[Arkan]

Holy shit, half you people are giving fucked up advise to this young lad.
Jesus christ, why take it out on the other guy? He's a guy and guys like to fuck girls....simple as that! She's the one who fucked you over, not him. You should've went up to them and said to the guy "Congrats, she's your headache now".
At your age, you need to be free, not tied down with someone who you think you're gonna marry. That shit never works at that age. Just go out and bang her best friend, making sure you bang her really good so word gets back to your ex. That'll piss her off more than anything.