I know it’s difficult. I separated from my partner about the same age as you, having been together the same amount of time (but for a different reason). It. Was. Hard. At the time I felt ‘too old to start again’ (oh how I laugh now, but I genuinely felt that at the time) and that I’d put too much into it to split up (sunken cost fallacy). But we did split up and it was the right thing to do.
Years after that I split up with another long term partner who had cheated on me. I tried to make it work, but that destroyed me worse than the cheating. It was awful.
Being drunk really isn’t an excuse. I’ve been hammered and yes tempted by some bloke, but when you’re in a relationship with someone you love, no matter how drunk you are, you still know what you’re doing is wrong and a choice you’re making. He actively chose not to give a crap about you or your relationship...
Yeah he’s stopped drinking (though I’m sure that won’t last either), but that’s not addressing the issue. That’s not why he fucked another girl. It made it slightly easier to lose his inhibition, but that’s all.
‘I want to change’. It’s a bit lame don’t you think? It’s like he’s going to ‘try’ to behave. For god sake! It’s not difficult not to get naked and fuck someone else. It’s not like accidentally biting your nails.
Personally, for me now, if someone cheated on me, it would be over because there’s no way on god’s little green earth I’m putting myself through that again.
I think you should leave him, you deserve better, but we are here for you either way..