First, I'm wondering how old you both are?  Second, if it is a long distance relationship, it might not work.  If there is no ring on her finger, I think she feels she doesn't have to answer to you.  If she already admitted cheating on you, I wouldn't be surprised if she'd do it again.  This sounds like a rocky relationship and in my personal opinion, I would move on.  There are plenty of nice girls out there.  Start over and forget the past.  I see more problems coming.  Sorry if I hurt you, but you asked for my opion.  Good Luck!