Hi Wolf - like everyone else here I am sorry we had to "meet" under these circumstances.
My WH is not a serial cheater in the sense of the word here, but he was a serial liar and he definitely cheated - lying about everything and anything, sometimes in order to hide his own feelings, sometimes to avoid conflict, sometimes because lying was so natural to him that he did it without thinking for no clear reason.
My experience and advice mirror each other here - my advice, and my experience with him, is that my WH did not start to change until he wanted to - FOR HIM. Not for me. Not for the AP. Not for his family, his friends, his coworkers - for him. Nothing I said or did, or I believe could have said or could have done, was going to make him want to change. He had to hit his own version of rock bottom. For my WH rock bottom was my leaving, him being outed at his work (his A was a workplace affair - he worked with the AP and the OBS and had been very good friends with the OBS before he decided to sleep with his wife), his being outed to his best friend (whose wife had an affair and left him so he was not sympathetic), him realizing that he had screwed up his home life, his friendships, us...and that he was going to be left alone with it all...and he was...and he didn't like it - didn't like himself. And wanted to change.
3+ years of IC later he is still working through his why's. I don't know all of what he has discovered as I didn't move back in, but we are rebuilding our friendship and we date occasionally now. He is still moody. He is still defensive. He is still insensitive. But, he's much more aware and is really trying to work on those reactions. He is definitely more honest, and calls himself out pretty quickly. He is able to apologize AND realize when he should.
He's different. He did it on his own. It took a lot of time. But most importantly...
When he actually started doing the work in earnest, I could tell. I remember thinking in the years before that, during the A, during the false R, during the aftermath, that it seemed like WH was trying but I wasn't sure. When he started doing it for himself, it was unquestionable to me. It was obvious.
So what I'm saying to you is - you have already tried the path your WH is asking you to go down now - you have been there, done that, and here you are now, right back in the same place. I did too with mine...3 times. You have to try something different at this point - for YOU. Maybe your WH will figure himself out, and maybe he won't. To be honest, my money when I decided to leave was on my WH NOT doing anything but watching me pack, saying stupid pointless "sorrys" and a bunch of other stuff, and that he would fade away into the background of my life. If you had asked me when I moved if he would have made the changes he did, I would have bet everything I would ever earn for the rest of my life that he would not. You can't know what your WH will or will not do going forward, but you can make a pretty educated guess about what he will do if nothing changes. I think there is a large part of you that knows that too. There was a large part of me that knew it for a long time but I tried to push that aside. By doing that I lost the one thing you can't get back - time.
I think you have to decide if you can live like this going forward. You indicated that you had good times you would lose too - and so did I. My WH and I shared some good times that I can recall even during his A and the myriad of lies he told me, but now, outside and away from that shit-storm of a relationship, I can see those times for what they were - fools gold.
So if you can live like this going forward - if "it" is worth these periods of unknowing, anger, fear, etc - than make your peace with that and accept this is who your WH is. There are some people on here who do just that - rugsweep, and stop playing detective. It sounds miserable to me, but to each their own.
If you want a change, I think YOU are going to have to be the one to do that at this juncture. Your WH sounds like mine in that he was not going to do much of anything, not for any extended period, to make a change for himself. His answer "I don't know why I went for it" speaks volumes. He needs to figure out why he would "go for it" - for starters. There is a LOT to that question - it's loaded. The finding out, for real, is not something that is going to happen overnight.
So ask yourself these two questions: "If I were to leave today and my WH decided not to fight for me - not to get to his why's and try to make changes for himself, would I ultimately be okay?" Or, "If things stay the way they have been for the last 15 years if I were to stay, is it worth it to me to stick around anyway?" Those are kind of the best case and worst case scenarios that YOU are in control of: leave and be okay (if you think you would be - I am not one to tell you one way or the other) or stay knowing this cycle is likely to continue forever. All the middle ground, the place where I am for example, required me to make a decision (mine was leave and be okay) and for him to do something as well (decide he wanted to change for HIM and actually do the work to figure himself out and try to be a better person).
I'm not telling you that you have to physically leave (although for me, after 2 1/2 years of staying it was clear that I needed to physically leave, for me) - but you can separate, or draw away from him for awhile and let yourself feel for a bit without his interruption. It's complicated, but it can be done.
It also stinks - the decision to go, the decision to stay. But it does get easier for sure! If you want out of infidelity you have to get yourself out of it. Your WH may follow, but the choice really is yours.