How do I win my ex back?

I'm going through this exact same situation. Broke up with my girlfriend of 2 years 4 months ago. After a month without her I came running back, realizing that I made a huge mistake. Nothing worked, especially since she found someone else (rebound). If you can, have a SINGLE conversation with her. Get your feelings out there, let her know exactly how you feel. If you can't find the right words, wait until you can. When the time is right, you ask to just talk to her. Send her a card, maybe some flowers..nothing extravagent. Remember, you broke up with her, she no longer has that trust with you, you broke that trust. She can only make the decision to get back with you, so do what you can while you can and some times....there just isn't anything you can do.