What do i do if my love of my life got a girlfriend that was my best friend??

well im in this situation where i am in love wit this guy an my best friend is datin him now!! an i want to beat her *** but i think it would be wrong but then again it want be because she shouldnt be messin wit him in the first place!! right?? well the guy an i use to talk an she knew i loved him an then she gone start messin wit him!! an well me an him is still friends an i miss him so much!! can yooh please give me some advise??