Well, having been the cheated one, let me offer what I can. My advice is to come clean. If he knew nothing about it, I'd suggest not telling him. But it sounds like he knows in his gut that you did sleep with him, and it will continue to knaw away at him. Tell him what happened, how many times it was, and why it happened (you mostly already have). Will he go a little nuts? Yeah, probably, but your voluntarily coming clean, even belatedly, will help him to eventually believe you when you also tell him that it will never happen again.
You need to stress several things. 1. It wasn't about the sex, it was about the support you were getting from the OG. 2. You take responsibility for it, know you were wrong, and are extremely sorry that it happened (you'll need to say this often over the course of the next days and weeks). 3. You no longer have ANY interest in that guy, or any OG. 4. You want to work on your M, and want it to work out with him.
Answer his questions but try to avoid giving out too many details about the sex itself. My W refused to answer certain questions and I now am thankful that she did.
His ego is going to be hurt. It would be very helpful if you told him that the sex wasn't that great and that sex with him is better. It might also be helpful if you would be the one to initiate sex more often in the coming weeks (to the extent that maybe you don't now). Make him feel sexy and that you want him, and he'll get over it more quickly.
Others here may disagree with my notion that coming clean would be best. I'll be interested to read what they have to say. Best of luck to you, and let us know how it goes.
