The day after my X left to go live with OW, a friend for church met me for lunch to help support me. The first thing I told her when I saw her was, "I've decided that when he calls wanting to come home, I will tell him that he can't come home for a year; that I'm willing to work on things, but we have to go to counseling, he can't come home for at least a year, and that, obviously, he has to quit seeing her. And that after a year I will decide."
She was kind of surprised I had come up with all that already, and she said, "WHEN he calls, not if, huh?" I told her that I knew he would call, it was just a matter of how long it would take. And that I knew I needed a plan before he did, because I didn't want to be one of those women that just keeps putting up with stuff like this, letting him come back over and over again.
He left on a Wednesday night (while I was at band practice at church - he just left a note). I met with the friend for lunch on Thursday. He called Friday afternoon, crying, begging to come home.
I stuck to my plan. It was the best choice I could have made. I didn't want to D without having done everything I could to save the M. But I also didn't want to let him walk all over me either.
He did everything he could to try and change my mind... came up with all sorts of crazy ideas, including one where we would wall off the 2nd bedroom and bathroom, and he would use the window in the bedroom to come and go (we owned a ground floor condo). I stuck to my guns.
We worked on R for ~ 2 months, until I confirmed that he was actually living with her again and lying about it. He was away from her less than a week.
The point here is - I gave him a plan that would at least begin to give me some assurance that he was serious about our M and serious about being faithful again. He couldn't even give me a week, much less a year.... which showed pretty clearly to me that he would never be in it for the long haul.
You're happy with how things are now. Do you want him back? If not, then just tell him that the ship has sailed.
If you want to give him a chance again - make a plan. Have very well-stated and clear cut boundaries. Let him know what it will take to even have a chance, and stick to it.
In the end, you have to do what's best for you at this point. He's already shown that he isn't going to worry about you.