WTF is up with people?! Sucks super bad. The only thing you can do is be honest and hope that she trusts you.
I went through nearly the same thing, but I was the girl. I was really into this guy/dating for a few months, and then (what I thought was a reliable friend) told me that he had been flirting with her and told her that he was only with me to bide the time. So I broke things off. He said it was all lies and what not, but I went with the chicks before dicks things...
Anyway, a few months later, he asked to talk to me. I agreed. We met up and he said it was all a lie. He never had interest in her. He had been missing me the entire time and was sad that I was seeing someone new, but he wanted me to know that he had never meant to hurt me and was sorry that it all happened the way that it did.
I was stunned and amazed and believed him 100%. Cause if you had cheated, you wouldn't care this far out or you would known that it was your fault and wouldn't still try to be with her.
Call her. Tell her you'd like to get coffee or something and be honest again. Unless she's an idiot, she will believe you.