Yeah, I get this but I at least give her credit for telling you. Some people keep this stuff a secret and it ends up being expressed through other actions and both of you start feeling tension.
Depending on when someone cheats, if it's early into a relationship I think it's easy to forgive that stuff because every has a past and everyone usually has multiple potential partners hanging around them. She cheated on you after you were married which makes it unforgivable. You've tried to make it work but it just doesn't.
Tell her this has been playing on your mind, you tried to get past it but you can't. It's time to move on. Just tell her the truth.