Let me ask you a question?
Who is your girlfriend to the world? Fucking no one.
Who is your bestfriend? Fucking no one.
If your bestfriend is man enough to take your girl then he is obviously man enough to fight you. Beat the living hell out of him and go find some better friends. Then when your girl comes crawling back because she regrets going for your best friend give her a nice fuck you.
FIND A NEW GIRLFRIEND YOUR OLD ONE WAS A WHORE.
Somewhere over 300 million people in the U.S and it's growing. Someone is bound to like you.