Girlfriend kissed another guy...what to do?

ok....so its like this....when we just got together...i had a friend who i thought was my friend....it was a girl and shes bi.......she asked me to do a lot of nasty stuff with her and i said no cause i have a gf.....she said 'so what?'.....i was like 'wtf......u hav no respect for my relationship and then i stopped talking to her to save my relationship cause i love my gf...apparently she went ahead and wrote on my facebook wall about what we 'did'....*sigh*......but i didn't know how to tell my gf.....until 11 months later.....and when i told her she got upset and thought i did the stuff with her but i didn't :'(.......i would never hurt her for anyone or anything.....so my started talking to this other guy...to make me jealous i guess....i even saw a text on her phone from the guy saying 'are you still horny?'....and i asked her wtf and she said its none of my business.......she continued to lead me on and talk to the guy for 2 weeks.....i wrote her a lot of poems(not internet poems, the poem came from my heart) saying i was sorry:'(.....and she ignored it........in the third week...she told me she had unprotected sex with him and he came in her and about his dick is the biggest and its 11inches and stuff........ she made me believe that for 4 days and then she told me the whole thing was a trick for me to see my mistake....i didn't do anything!!!!!!!!!.....so i kinda started to have doubts about her....... and then later she said...she didn't have sex with him she only kissed him once.....truthfully....i dont know what to believe.....i dont know what will come next......