allfalldown, I so feel for you and can very much relate.
My husband lost his dad to cancer, fairly quickly after diagnosis. It was during that time that she "snuck" in. She was "there for him". He said I wasn't, which crushed me. I tried to be there for him, but he wouldn't let me. He was pushing me away while letting her in. This was the build up to the affair.
The affair initially lasted only about month before they broke it off and he told me about it. That was July.
In hindsight, I wish I had known what I know now when my husband was in the "fog" as they say. If you haven't read the article on the WS Fog, you should. It describes where my husband was at to a T, including the "addiction", which is the word he used when he described it to me later. What it doesn't mention is to be wary of him going back to her, which is what mine did. Emotionally, he never really broke it off and only a handful of days after Dday he called her. When I found the phone call on his cell bill he told me it was for closure, but I found out months later it wasn't closure at all, it was "goodbye for now . . . we'll figure out how to connect again soon".
I knew he was still "supposedly" in love with her during that time (now he says that he realizes he wasn't, so maybe that will be of slight comfort to you). He was certainly depressed (as he was prior to that when, like you, I thought it was about his dad dying and/or our finances.) Just as the article about the fog mentions, my husband told me he loved me but wasn't "in love" with me. While I was doing everything I could to love him and was "mostly" confident he would fall back in love with me eventually, he apparently couldn't imagine ever being in love with me again. (He didn't tell me that at the time.)
All that changed when her husband found a message from him to her on Christmas Day and I got my second Dday. Merry Christmas to me. However, that night my husband was able to truly resolve to let her go and figure out how to love me. Within only a couple of days he had fallen in love with me and it only got better from there. Better in the sense of him loving me massively and my love for him growing even more. My pain is still intense, even more so from the many months of betrayal after I had forgiven him and was pouring my love all over him.
While every situation is different, I believe, and my husband would back this up, had I followed all of the behaviors in 180 article, it would not have been helpful for our situation. Some of them make perfect sense, however, because of our history, and the fact that my husband didn't feel loved by me the way he needed it prior to the A, had I not shown him how much I needed him, and how loved I could make him feel . . . had I threatened to leave as some people suggest, or actually left, our marriage probably would have been over. In his mind, a happy marriage for us was hopeless. He couldn't imagine it in a million years, so me acting like I don't need him and not lavishing my love on him would have just cemented this in his mind.
As hard as it is to look back at all those months that I showered him with love while he was apparently still seeing her, he says he believes that's what saved our marriage. It was during that time that she slowly began to lose her grip on him. He began to realize how ridiculous what he was doing was and how he'd been rationalizing everything. He was slowly coming out of the fog.
I guess the best advice I could give you as a person who's been there is to not lose hope. It hurts like hell knowing he's "grieving" another woman. It hurts that he says he's not staying for you, but I believe while my husband didn't say this specifically, he probably thought it. I think he initially stayed for the kids and because of the history we have and it was the right thing to do. He did love me, as he told, but wasn't in love. Ouch.
And yet, he came to be so madly in love with me, even more so than ever before. He told me he's never been in love with anyone but me (which is saying a lot since in addition to the A, he was engaged to someone else before me). He could finally see that what he felt for her wasn't love. He even began to have anger toward her because she wouldn't fully leave the cycling group he and I were both a part of. There was a time when I felt he was defending her right to be in the same group I was in (when he was still seeing her), and after he was truly out of the fog, he realized it was ridiculous for her to be there and that she should just leave me alone.
I tell you a lot of details of my story in hopes that it will give you hope that it can get better. He can let her go completely and fall madly in love with you again. I also tell you that he went back to her (never really left) to keep you aware. I had zero clue he was still seeing her. I had done a lot of snooping for the first month or so, then a little less until I didn't really check up on him at all. I couldn't have been more whacked upside the head when he told me on Christmas night. And I feel so stupid about it now.
Having said that, I also fear what may have happened had I found out earlier, when he was still so attached to her.
At any rate, maybe in your situation the 180 thing makes sense. For me it didn't. Hopefully my story is a help to you in some way. Big hugs to you during this very difficult time.