Noah, Its a very hard thing to have to deal with. Its to bad it had to happen to you in college and not in high school, strictly because you can have some serious down time after it. It happened to me my first year of college, I wished it had during high school. It shook more for about 3 solid months. That was just for me to be cool, but it hurt if I saw her for well over a year.
Step back and step away, try not to see her ever again, resist the urge to call her, block her on your IM lists. Avoid contact and start healing. Surround yourself with your close friends at all times during the initial period. Keep your alone time minimal because you mind chances are will torture you with situational based thinking. That being the what-if, if I had's, whats she thinking now. The bleek part of this is there is no hoppe to getting her back, the battle is lost, and honestly who wants damaged good if you've been a consumer for 2.5 years.
Dont do anything rash to the other guy. I know the urge is there but take it from me, the repercautions are not worth the actions. Chances are they will call the cops.
Once time has progressed, you will be able to deal with the thoughts in a rational manner. Realize there are many women out there and most definitly better ones than you had. I had to figure that out first hand after my high school ho of 2 years did the same to me. Except I got the pleasure of knowing she was banging some guy for about the last month of our realtionship.
Again dont do anything rash, not to yourself but to them, its not worth it.