How do I make my ex come back?

My ex and I dated for two years. She was my first relationship, my first everything. We were inseparable. I love her more than anything and she means the world to me. A couple weeks ago she went to visit a college. She had told me though "don't worry I am not going to do anything with anyone. You are the only person I love and I am not going to ruin two years over one dumb night. I promise I am not going to do anything tonight." So I believed her and trusted her. I had no reason not too she had always been faithful. I call her around 1 a.m and asked how everything was going. She told me " good, I just got done hooking up with some dude." I hung up and say outside the rest of the night. Long story short, her and this guy talk now. They started talking a day after she cheated on me. How can I make her realize she just lost the best thing in her life and make her run back? She has not tried to contact me once since the incident.