How can I get her back?

Ok, this is a tough situation. People will do this occasionally to someone, they'll always lie just to break them up. I think you should find out who told her and then figure out why they said that--your ex could be spreading rumors about you. Second, you need to go to your girlfriend and talk to her. Sit down and explain things calmly. Show her that you do care about her more than your ex. Send her flowers, little cards, act like you are re-courting her. Show her that you want her. Make her feel like she is on top of the world. That is the ULTIMATE way to get her back. To make her feel loved and to show her that you do love her.

This is a tough one. Maybe she just feels like she can't trust you, so really show her how much you care. Do something romantic you have never done before. Let her know how much you want to be with her and tell her how you really feel about her.