Lolobug
It was a terrible stupid thing he did, but the deed is over. If he is sincerely sorry and is trying to mend his ways, then his side of the street is now clean.
What that leaves is you and the hurt that won't stop. Obsessing is making you sick, and I think you realize that. You can't change it and all the "Why's" and "What If's" in the world won't make it better.
I believe that the antidote to resentments is forgiveness. It doesn't mean you have to stay with this man if you are not happy, nor that you can't move on with your own life. It means that you turn it over to God and let the poison out of your heart.
Then, when the thoughts come, have a plan. Repeat the serenity prayer, pick up a good book, take a walk, let the thought cue you to do something positive for yourself.
If none of that works for you, maybe get some counselling for yourself. I'm not including him in this suggestion, although joint counselling might also be good, but counselling just for you may help you work through it. That said, I'd still put my money on forgiveness.