Having been with my bf for 4 years, that would be really hard. Since I love him and have always planned on spending forever with him, I would try to get over it. BUT, I have never ever thought he would ever do anything like that, so it would seriously make me question how well I know him. Plus, i am a jealous person so I don't know if I could ever get over it. It depends on if it was just a weak physical moment, a long term real relationship, or something else. Something physical would be a lot easier to get over, but if he slept with her, i could never get over that, because we are the only ones we've ever slept with. I bet I would be surprised about how I could get over it though. We always seem to get through anything because we love each other so much.