Shit, I found out from one of my ex girlfriends, that when were were messing around she was also messing around with two of my friends. These guys aren't the best of people, but they're the type that are fun to party with. She then chose me over them, and we were together for about 3 months. A conversation of past relationships came up, which always makes everyone feel like shit, and that's when I found out she was ****ing my friends at the same time as me. That's the kind of shit that hurts more man. Closing your eyes at night and picturing your own buddies (knowing what they're like), going to town on your girlfriend. Then realizing it was in the same time frame as yourself...
Needless to say, I was an alcoholic for a while then moved on. If I can get past that, you can get past something that happened before you even knew the girl.