Dude I feel your pain. Same thing happened to me. Like the exact same. I was engaged and it was basically the biggest kick in the balls. I just ended things a few weeks ago because I just didn't trust her anymore. My philosophy is this. Forgiveness is good. But you don't have to stay with that person. You gotta be happy and part of that is having a girlfriend that you can trust won't let some other dude into her pants.