Welcome to SI. I'm sorry you qualify for membership, but since you do, I'm glad you found us.
I see lots more gray than black and white, and I'm happy about that. Are you happy about being a black/white person? You may be staying because you're thinking there may be some gray in the world. Maybe deep inside you're thinking it may be worth some drama to be with your H.
I do not mean that I think you should R. I mean that, IMO, your best bet is to ask yourself new questions and try on some new attitudes for size.
I have really struggled these past three years and feel as if the only way to heal is to leave him.
Gently, it's possible to R and really put your H's A behind you. My reco is to give yourself permission to leave and to stay. You can't really say 'yes, IMO, unless you can say 'No,' too.
I'm sorry you're struggling. You really do have a difficult decision to make - you've got decades of your life depending on what you do. You've been traumatized. Big decision can lead to struggling.
You may really want to leave, but the fact that you're not giving yourself a choice may be holding you back.
But, as soon as I get to that point, I question whether his grief and the approach/opportunity really was the perfect storm.
That's about him. Mitigating factors don't change the fact that you've been betrayed. Sympathy for your H is separate from your need to decide what you will do.
Not so BTW, kissing is physical.
Am being a cold-hearted, selfish bitch by not giving grace?
You know the answer better than anyone here does, but choosing to end an M after being betrayed is totally, absolutely, eminently OK.
Some ideas to try out:
Nothing you did or didn't do caused your WS to cheat.
Focus on your own healing before forcing yourself to make a decision. The better you feel about yourself, the better the decision will be - and being betrayed is a major attack on one feels about oneself.
Being kind to yourself will probably lead to a better decision than beating yourself up.
When you notice yourself blaming yourself, you'll probably help yourself by figuring out what you're feeling - anger, grief, fear, or shame - and comforting yourself.
At the top of the 2nd page of the Just Found Out forum you'll find a number of threads with bull's-eyes next to the title. I urge you to look at them. They offer a lot of good info that may help you figure out what you want to do.
Also, the 1st thread in the Wayward Side forum may help you identify how good a cadidate for R your H is.
My last reco is to make surviving and thriving your goal. Then choose the best way to do that.