Its been a year and 6 months how do i deal with this girl?

so i met this girl in one of my classes last year and we became good friends then closer but never bf and gf but her ex was a 24 year old guy and she was 16. i was 18 and the story goes she broke up with him and we started to get really close then i saw her texting him and saying i love you and cant wait to see you and stuff and she said it would never happen again and how she didnt want to lose me and blah blah but i learned she still talked to him just changed the name of the contact in the phone. i took her to prom and ask her out and she said yes but told me the next day that she couldnt do it, she became really distant and i was texting her one night trying to cheer her up when i got a call and alll i could hear on the other end was them having sex, she swore to god she didnt kno how she called me but then we were off and on talking for a few months then in the summer we had a big fight and i said im done then after summer she came back and said she needed to tell me something so i met here and here and my best friend had sex, i again said bye then a few months later started talking again, she texts me outta the blue and said she needs me and that she might be pregnant because guess again her and her"ex" had sex so i freaked and a huge fight started three months went by and she again texted me and said she loved me and she needed me in her life and all this **** which i bought but i kno history is repeating its self because we were ok for a while but shes there everynight and stays the night at his house....id be stupid to think they are just friends which is what she wants me to believe but i kno better and when i ask questions she says your not my bf i dont have to tell you ...i kno ve stayed way to long and its changed me asa person i mean my ex best friend moved in with all my other friends so i dont really have a place to hang anymore, i have the option of telling everyone so they kno her secret because i kno she is terrified of people knowing but will that really make me feel better?...and im not looking to get back with her or make her want me or something i just want to kno what to do next and make her understand...need to figure out the next step