I don't know that I have a lot to offer, but there are parallels here that I can't help but notice. And, I am trying to figure out the truth of my marriage too right now, and there are some things you said here that raised some questions for me personally.
A couple of things to know - I am the WW. My affair was 3 years and some change ago. I have worked really hard on myself, and thought we rebuilt. Now I recently found out that H had his own affair.
I am not bipolar but I was in the midst of emotional exhaustion before and during my affair. I was really not well, it was obvious to everyone. I think that my lack of wellness kept my husband around for a while, and he has maybe guilted himself over not wanting me to become unwell again. I was also sexually dysfunctional at this time because I was just so numb. I presented as wanting a lot of sex during that time but it was coming from the fear of "not getting my mojo back". I could not climax at all, not even while alone.
I am also very cheerful and energetic when I am well, and we were also married for decades at the time of my affair.
I think my husband was unable to unleash on me all of his feelings. I am not excusing his affair, I know he had other choices. But, I think he felt if he swallowed it long enough and faked it until he made it we could be okay again.
Part of me feels if he had talked to me about what was happening in his interior world, that while I could not change the past it might have helped somehow. Things tend to gain a lot of power when they are held in and only examined by you in your own head. It actually becomes your own secret and separates you further from any intimacy that you might be able to reap in the marriage.
If she is better, can you talk to her about this? Perhaps even think about doing some individual therapy too. I tend to think that my husband didn't feel completely like he had a right to all his anger because he knows that I was not in my complete right mind while it was happening.
However, I should add that I did in fact take full responsibility and never really used the mental issues I was having as an excuse either. I thought that was best, but to him I was just so into my own self flagellating that he couldn't bring him self to add to it.
I am being a bit generous in my description of him but these are early days after his disclosure and I oscillate between condemning him and understanding him. Your post forced me to examine a little more this aspect of our recovery.
My advice is if you want to feel better, there are ways you can at least strive for that. It might mean unearthing old artifacts, and it might mean embracing solutions you have wanted to avoid, but nothing is worth staying in some grey zone on your own healing. You deserve more than that in life.