I can relate in many ways to this, if it's helpful. It will be long, but maybe parts will touch on what you may have felt. I know the situations are very different, but sometimes with these complicated messes there are similarities in the midst.
My WH had an affair with a work colleague / neighbour and we also had three D-Days over six months of R and I'll explain a little of what was going on so there are maybe insights for you also.
We were separated by distance (he was working at home, I was away studying for two years) and the pandemic hit and we could not visit in the end for over a year. She was both a close colleague and lived more or less next door, and she was interested in WH from the outset and was obviously persuing him, which he didn't really as she was old enough to be his mother and knew he was married.
Over six months of on an off lockdown and working from home they got very close as friends (all her instigation but he did little to resist as he says he was lonely, stressed and no one else was around). Psychologically classic I guess - she presented herself as a Mamma figure and love bombed him. Very kind and caring, very supportive, asking lots of questions and listening to all his dull problems about work. Miraculously (draw your own conclusions), she liked all the same things as him. He loves scrabble and plays on his phone all the time. OMG, she LOVES scrabble and can they have a tournament please? He likes Bluegrass music. OMG, she LOVES Bluegrass and does he want to come over for lunch and listen to music. HE works out 5am every day. OMG, she LOVES working out at 5am, and how about they do it together????
Basically, I think she just faked the entire thing to become his best friend as she fancied him as a partner, because honestly no one likes bluegrass or 5am workouts?! Surely. Anyhoo. WH is pretty clueless so I do believe all this just slipped by him and he thought this was just a great friendship. The line got crossed with emotional intimacy. Again, classic psychologically. She presented herself as "abused" by her ex, damsel in distress. Made him (who had an abusive childhood), identity with her. Then he started sharing his deeper thoughts and feelings, which really WH doesn't do with anybody.
Boom. It's an EA. Not romantic in nature, but very close intimacy and basically every day spent together. She offered emotional support over coffee, dropping over food, keeping him company over lockdown for movie nights with all HIS favourite movies of course, wanting to spend every minute with him and showering him in adulation. She also showered him in alcohol, and he was never a drinker but with her it was alcohol all the time. After six months of this, when drunk, he slept with her. Then began a PA.
It was a PA in the sense they were basically doing couple things all day every day, and getting drunk and sleeping together once every week or two, but he never sent her lovehearts of professed any feelings. It was more like FWB. She told him she loved him and and he said he loved me and didn't want to have an affair and just wanted to be friends, but he didn't stop seeing her so the sex kept recurring. I have asked why, and he said he doesn't fully know (hoping he will figure it out in time). I think he didn't want to let go of this amazing friendship he believed he had with her which made him feel so understood and special.
There was a big element of her creating a strong emotional dependency on her over such isolated circumstances, endless alcohol, him being depressed over the lockdown, childhood issues in him that made him crave external validation, me being gone for over a year...and he said there is also a big element of believing that as he'd slept with her once I would definitely leave him, so once he got drunk, the self-destructive part of him would think, what did it matter if he did it again?
Immediately after the PA began, she quickly turned from fun-time-Sally into a fruitloop from hell. You name it, she did it. Threats to call me. Blackmail. Hitting him / herself. Drunk driving. Public tantrums. Breaking into our house / car. It was bonafide fatal attraction and she could not accept after them spending so much time together every day for 6 months that he still loved me and didn't want her. All the while my hapless WH tried to keep going back to when they were "just best friends" and thought and believed he could achieve that and somehow save the situation he had created.
It blew up in his face after four months of this when she called me. At that point she had already attacked him multiple times, demanding he leave her for me, and he was on pills for stress. After that call, when we started attempting R, she basically stalked us both for several months. Letters, gifts, hacking into my social media...we had to call the police multiple times. It was bad.
Although he stopped seeing her, he also would not properly stand up to her, and everything was about "she is unstable and threatening me so we have to do it her way", and it was a huuuuge mistake to do that. I wanted him to make a complaint at work about the stalking, he refused claiming he was worried about her following through on threats to ruin him. I wanted him to block her on social media, he refused, claiming this would make her angry and she'd only come and break into the house again.
I think, looking back, part of him was genuinely terrified of her because it was obvious she was crazy, part of him felt responsible for her and guilty because he thought he had made her that way and part of him didn't want to lose her attention (even if it hurt me) and part of him wanted to still believe she was his best friend and all the good times were real.
With us still long distance, and pandemic travel still extremely hard, this made R almost impossible. He was still seeing her every day and not seeing me, and his boundaries with her were far too weak. At one point she was coming into his office crying every day asking to meet and talk and he was refusing to stop her. Our R became VERY rocky as a result, obviously, and despite his efforts (counselling, reading the books, supporting me) there was no way I could R with her rubbed in my face every day and him seemingly not stopping it.
This made it false R really, because he was not really doing the full work needed to make me feel safe. As things got rocky with us and our R, and she continued to stalk and blackmail him, he unravelled mentally. She was putting SO much pressure on him to meet up with her to give her comfort, and he was coming home from work in the middle of the day sick. Meanwhile I was getting angrier and angrier and I ended up telling him I wanted a divorce and having a lawyer send him a letter.
That very day he got the letter, he went out, got blind drunk and ran into the psycho and slept with her again. The next day, he confessed to me and that was DDay 2. I cannot explain how bad that was for me. It was so much worse than the first DDay, that he would do that with someone who had been frightening and attacking me and making me sick. I think he just ran to his known source of love when I rejected him. In his weird, WH logic, the stalking just proved how much she loved him and even though he didn't return her feelings, it was source of validation and comfort to him when he felt like a piece of shit.
After that, he got very ill almost immediately. Panic attacks turned to depression and constant crying which turned into more or less a complete breakdown with paranoia, disassociation and almost catatonia. He lost about 25lbs in a couple of months because he stopped eating, and he quit the gym and started drinking a litre of spirits nearly every day just to function. That was awful to watch but I was still far away. I helped him a lot through that, because I was genuinely scared for him but strangely at this point he was both absolutely TERRIFIED of AP (I mean he would literally be screaming if she called and threatened him, which she frequently did), and also simultaneously missed her friendship and how things were before the affair started and blamed himself 100% for all her behavior. Weirdly, he saw her as two people - one the terrifying psycho abuser, and the other, his kind and lovely best friend.
Over 5 months of him being very unwell in an obvious way, AP manipulated the entire situation to try and harm him. For example, she started to call me to tell me she was worried about him being mentally unstable and tried to befriend me and turn me against WH. She was very convincing and even I believed the concern was real. The AP was honestly the master manipulator! After months of stalking him and breaking into his property and desperate and insane behavior, SHE told their boss SHE was concerned about WH being mentally unstable and suggested he be put on mental healthcare watch!!! She spread gossip about WH being insane in the community and with fellow colleagues and spread the narrative that this had happened because he was so in love with her.
All the while, I was supporting him long distance and doing everything I could to help him. R was put on hold completely while he tried to recover to some level of function and I was completely selfless in that process.
We realised we were never going to be safe from AP, so we decided it was better if he gave notice at his job to move to be with me, as my degree was coming to and end we thought complete relocation was the safest idea. As he began to get better and went back to work, he and I were separated by distance still for another few weeks, and although he still had to work with her, he was meant to be in NO CONTACT on a personal level.
After 8 months of R and not exchanging a single message with her over that time, he did it again.
She began popping in for coffee at work again and expressing concern and how much she cared about him. He allowed that to happen, because he said she had a new boyfriend now and he thought "everyone had moved on" and he wanted to close the situation off nicely before he left for good. She was blocked on everything by now but started sending him loving emails to his work account expressing how sorry she was for all she had done, and it was just because she loved him so much and reminiscing over their wonderful friendship and how special what they had was. For the first time since R started, he his this communication from me and instead of deleting it, he responded, saying he forgave her for everything and really missed her friendship but could never tell her that because of me, that he was sorry he had hurt her, and they had wonderful times as friends together and he was so sad he could not see or speak to her anymore after he left.
It blew my mind that he did this, but he did!
True to psycho form, she immediately screen shotted the emails he sent her, send them to me, and voila - DDay 3.
I think it was partly down to him still being physically distanced from me and seeing her every day still. Partly down to him being in a feeble state and her basically acting all kind and nice again and making out to be no threat. Partly down to him genuinely believing they had a real friendship and feeling grief over losing it. Partly down to just not having done the work of R to realise that lying to me again and communicating with her like that was a betrayal worse than sleeping with her.
He came home a few weeks after that, and for a while AP continued to stalk. He deleted all his social media accounts as she was setting up fakes. He changed email, phone numbers and jobs. It took him about 3 months of being around me and away from her for his depression to lift and he is 60% better now. AP even came on "vacation" about 3 miles from us! She is a psycho, no question. We both have ongoing fears over what may happen with her, but like you, I have some confusion over how FOR SO LONG he allowed his boundaries to be crossed.
We have a long way to go with R. I don't get how he betrayed me as much as he did over such a long period, but I need him to do the work when he is better. Things are definitely rocky, but any feelings of attachment he had for AP are finally gone now. I think the truth for every affair with an emotional connection, is that it is hard to break. The affair is meeting a need which they find hard to escape from, even if the AP is crazy.
I know the big question is, "why did he not get this need met by ME", and I think the answer is that people who have LTAs are generally a little bit damaged or vulnerable in some way to needing things externally. They're not necessarily possessing of the skills they need at the time to make healthier choices, like self-awareness or empathy or calculating consequences. They just have a need and get it met.
I don't believe my R even began until the last time he saw her, so for me it's been a year since DDay 1 and about 3 months since R began, and R is a hundred million times harder than it would have been if he had not betrayed me after the first DDay. The truth is, he was not ready to give up what he was getting from AP. Even if it was toxic and scary, he was needing it, and when he was weakened by shame and doubt about us, he was more vulnerable. He needed to get himself in complete no contact before he could even begin to work through it himself.