Lying Cheating girlfriend?

Okay, im going to try to make this as simple and straight forward as possible... First of all, we do not go to the same highschool as eachother, and we do most of our tallking online (text and video) and some together. So... Last friday, she just got off of her grounding for a week becasue she got high at school and was hospitalized becasue she couldnt walk or do anything. All weekend, she cried to me on the phone telling me she would never do it again, and that she was quitting the drugs, and she was never going to do anything to the relationship that might hurt it. Well, it was good for about 2 days. Monday came along, and when i was home from school, i got a facebook post saying she was high again, and grounded for another week, and we arent allowed to see eachother for over 2 weeks. So i was talking to her friend online (her best friend) and asked her about the situation, and i cant remember how, but we got on the topic of her telling me everything that my gf had lied about. There was an incodent where a guy kissed her. her story was that she didnt kiss back (truth, she did) then told me that my gf actually never quit ciggarettes or marijuana,a nd both of them were confirmed from her step mom... so there were a couple lies already. then i remembered a story she told me a few months back about a "small party" she went to, and played strip guitar hero, and she claimed that she didnt play and left early. i was supicious, and so i asked her friend about that night, and she told me that my gf agreed to cheat on me, made out with this guy, then he fingered her, then they played strip guitar hero and got right nude for eachother. When i brought this subject up to my gf, she argued that it was false, and she would never cheat, and the next day, i really got annoyed, and told her to tell me the truth, and she confirmed that the guitar hero "party" was true. before this, i didnt think she Smoked, did drugs, hung out with the people she said she didnt, and then i find out she cheated... i dont know what to do now... can anyone PLEASE help me!!?!?!??!?! =( i feel soo upset and broken hearted... and i cant really talk to her cause shes grounded again... but she might be on in 2 days to read my emails... what do i do... what do i say... im lost with this, and feeling horrible. we have been together for a year and 1 month almost. ive been there by her side when things were bad, and when they were great. we spend every holiday together (including christmas) and we are great together.. well, so i thought.. i need some advice.. =(