You never said that you for sure knew that she was cheating or you. If you can't get a straight answer from her about something as important as this, it sounds like there are other trust issues.
If anything, I've learned that no matter how much you want to, you can't control another person's actions or feelings. You can only control whether or not you decide to continue with the relationship.
Also, if any guy let me cheat on him and waited patiently for me to make up my mind, I prob wouldn't stay with him anyway.
Trust and respect, dude. That's what make the marraige-go-round.
Best of luck...