I know affairs are from loneliness, fear or anger (or all 3)
Um, what?
Affairs are from low self esteem and entitlement, poor coping skills and pure selfishness, not loneliness, fear or anger. Well, I guess you may find some revenge affairs stemming from anger.
Your H wasn't lonely or afraid. He was selfish and has a low self esteem. He found someone to boost that esteem by accepting him and wanting him, and he went for it to get a 'fix'. Don't think for a second this was some kind of cry for help. It was an act of pure selfishness, nothing more.
Alex, I realize you had 20 years that were 'good', but your H is now a serial cheater. He won't stop. He's done this more than a handful of times, and honestly I'd be looking really closely at the first 20 years of your relationship. It wouldn't surprise me at all to find more cheating back then, and that you only started finding it after you started looking, once he was caught 4 years ago.
Your H does this every time. He cries "oh, you deserve better. I won't stand in your way. You should be happy, I'm so sorry but I understand, blah blah blah." This is what he did last time, no? He's now trying for your sympathy, but I've yet to see any real change from him, and neither have you which is really what's important here.
He's suddenly willing to let you go. Perhaps this last one was an exit affair, where the WS cheats hoping that the BS finds out and ends the relationship. That way, the WS doesn't have to sit down and explain that he/she just wants out.
Alex, you aren't 'throwing in the towel'. You are standing up for yourself. You've been through so much in just the last few years. You really don't need any more grief, and if this was an exit affair, that's what you'll end up with - because he'll do it again.
Aside from the affair, what does he say he wants? Or is he only saying he 'understands' why you want to leave? I'd be very curious where his head is relating to that question - does he want to R, or is he also 'done'
Your posts are so filled with pain Alex. I feel like your WH is manipulating you, saying he understands why you want to end things, leaving you to either not end things and hope it doesn't happen again, or end them wondering, since he's being so nice and understanding, if you made a mistake because THIS time may be the last time. He's putting it all on you, when it should be all on him.
Alex, do you really want him, after all he's done, or is this such a big change that fear of the unknown is really holding you back. That fear is very normal, and many people do stay because of it. A major change after nearly a quarter of a century is really a big step.
I think you really need to see where his head is, and what he is willing to do to if you stay together. Is he willing to do the real work this time, or is he being sad and 'letting go' because that's what he really wants but doesn't have the strength to say that?