You need to be sure that she wants you back. However, I think that you should let her go. If you want to get her back, you can try talking to her and express yourself to her. Get some more details on http://www.ehow.com/how_2004302_get-girl-back.html.
She broke up with you for a reason. It might be that she was already hooking up with the other guy when you were still a "couple". Cut your losses, move on. They're is always another "love" headed your way.