Dude, you r screwed! The best thing I can suggest you is stay away from her for at least a year and also do not hook up with other girls. Maintain a jogi lifestyle. Try to meditate. Visit temples often. Go for jogging every morning. Do not try to think of her, neither that guy nor what happened when you were away. Stay on low profile. Keep yourself busy with work and school (if you r enrolled). If even after a year this incident haunts you, then you can't get over to it. Look for another girl and try to be honest with her. But I doubt you are not gonna get a first hand.