Seriously I need advice....Desperately...?

This is almost exactly my situation. Except I like the girl, and they like eachother, and I resent the guy for what he has done. There is really no proper way to act. No matter what, people will get hurt. It is likely that you will be hurt the most. You must be able to accept this though otherwise the situation will explode or you will stay in a rut forever. I am sorry I can not give you an exact code of conduct, but there is no clear way out of these sorts of situations.