To me, this sounds like you're being used. (I also go by the assumption you're a guy. Hard to tell from your post, srsly!) Obviously you can argument otherwise as you have been a friend with her for so long, however what she has done is completely... how to put it... cheap. Even IF she likes you, kissing you while having a boyfriend is absolutely out of the question. It's understandable you'd go with the flow at that point, but you really should give it some extra thought, now that she has cheated on her boyfriend with you.
As alot of the other posts are mentioning: you should be careful with this girl. She sounds like a proper femme fatale who can drag you all the way down. Don't go that road. I have personally been there, completely blinded by love to this girl, and the only thing I got out of it was a depression and grey hair. Shut her off. You will feel it is wrong, but I can assure you it is what's right. You will appreciate it some day.