I have been in a similar situation as you. I am so ashamed of it...
My guy wasn't a felon, but he was surely a dodgy character anyway. It was the passion of the whole thing that I was attracted to I think. It was just like the movies. I have never had anyone WANT me so much. He was a charmer and a romantic. Unlike anyone I had ever been with. But I still should have know better bc I had a wonderful marriage. Unfortunately I did sleep with him. And there were consequences.
I know you said your mairriage is rocky, but I don't think it should make any difference. I would give anything to take back what I did, but I never can.
You have stopped it and that is the first step. Continue to keep the ties broken with him. Don't go too far and live to regret it like I did. Stay stong.
Now you have to decide what you should do in your marriage. My advice is that it is always worth it to try to work things out.