OK, first of all, this is WRONG. YOU are not ruining anything. HE is the one doing the cheating, and yes, it IS cheating. HE is the one who is ruining things, NOT you!!!!!
First of all, you need to tell him what HE has to do if he wants you to stay married to him. Yes, you DO threaten divorce. That IS how serious cheating is. I can think of a few things:
- you and he both get STD tested (YES, because you have no idea if he did in fact sleep with someone - you cannot trust him)
- he has to get counseling - he MUST figure out why he cheated and fix himself so you can have confidence he never will again
- he has to give you full and unhindered access to his entire life, totally at YOUR whim and on demand. It sounds like he's done this, partly at least. You need to see his bank accounts, emails, computers, devices, credit cards, vehicle mileage, WHATEVER you need to see in order to feel better. And he'd damn well better give it to you eagerly.
- This access continues for as long as YOU need it to. He doesn't get to say when it stops - YOU do. It could be years - WILL be years - before you feel you can relax and start to trust him again.
- If a few months go by and he has proven that he's not acting out, then you consider marriage counseling with him. If you decide to go, HE finds an acceptable MC and makes the appts and tells them why you're there.
This will take years. I am 6 years out and it's been maybe a year and a half that things are really good again.