You are too attatched to her. You are hurting yourself and threaten to ruin your life.
She made a bad choice. You are a guy that seems to care for her. But you do NOT CONTROLL her. You CANNOT make decisions for her. Please do anything you can to take a break from the situation. Talk to your parents, and friends about these feelings. They can help you get through the pain. Take a deep breath, life is good and wonderful. Remember that she may change her mind. If she doesn't, I promise you that there is someone you will meet one day that will be better.
Above all do not hurt yourself. You are an important living being, and you deserve to be happy. It may not be now but it is coming. Just wait this out. You will be okay.
Agreed with Gelise, you are too attached. You aren't responsible for her and can't control her. It *****. Believe me, I empathize, but if you absolutely can't stand her hanging out with that guy, leave. She isn't right for you, but someone else is bound to be (I found that out last year). You know how girls are always told that they should stand up for themselves and not be with a guy if he hurts her emotionally? It applies to guys, too. Corny, but true.