How to fix this huge mistake with my gf?

It was one week ago, I went to my high school's lacrosse banquet for the team. My friend (who is a girl, which is where the conflict starts), needed a ride home. So I gave her a ride home, and I stayed at her house for a little while because she invited me in. Most important part: nothing happened!!! Anyway apparently she txted her friend that I did something with her behind my gf back. This friend she txted happens to my gf's bestfriend. So i get a txt from my gf that asks why i cheated on her..... I said what r u talking about? after that, no respnse. This was saturday, on sunday i tried calling and txting her, she didnt answer. The next day i saw her at school, i tried talking to her, but she wouldnt listen, and she told me she wants a break from me for a few days. I couldnt even tell her my story because she didnt want anything to do with me. She seemed really upset, almost like crying when i was talking to her. Than to make matters worse, her cousin goes to my school and he was saying all this **** about me and asking me why i did it, and i told him over and over i didnt do ****.....Than i hit him out of such anger, right in left eye. How can i get her to believe me, and how can i get my point across so that she will listen? Is she done with me because she wants a "break"? And what do you think about this other girl that made that up? And did i **** up by hitting her cousin, not on very good terms with kid considering i gave him a funny *** shiner, anyway, sorry, im so confused, thanks for an answer though.