He’s clearly having some kind of sexual relationship with another woman. It might be an online relationship, or it might be a real life relationship.
You tried to fix this once, but here you are, in the same situation. Did you do any couples counseling the first time? Or did he just promise to change his ways and leave it at that?
Do you still want to have the relationship? Or are you done? If you want to try to fix it, then you have to get into counseling together. There’s a reason he’s doing what he’s doing, and saying “he’s a cheater” doesn’t begin to explain why he is doing what he’s doing.
When I did that (including the underwear pix), I was desperate for closeness and sex with someone who really was into me, unlike my wife. I was too scared to ask my wife for what I really wanted. I thought if I asked her, we’d end up divorced.
We did end up in counseling and I had already told her what I’d been doing. Our therapist said that if it’s going to work, the guy (or woman) has to admit what he (or she) did. If they don’t, it’s just too much to get through. How any person can face such details like you’ve got and deny they are doing anything is beyond me. He must be horribly afraid of the truth. But what does he really want? Honestly, I have no clue. I eventually told the whole truth.
If you’re past that, then I think @RealEyesRealizeRealLies advice is pretty sound, so long as there are no kids. If there are kids, then you don’t want a horribly acrimonious divorce, if you can avoid it. Of course, many guys are real jerks, who won’t cooperate with anything, forcing you to take them to court, and stringing it out for years while they use up the resources.
So what do you want to do? This must be a horribly difficult time for you. It’s like a very slow death scene. You may or may not be ready for it. Oh hell, no one is ever ready for it. It’s just really hard. Line up support, if you can. Be prepared to be depressed. You’ll have a lot to fight through. Good luck. Come back here to ask any question you like. Maybe we can help.