Most Helpful Girl
You made a promise to the girl whom you "thought" she was. Not to the person she is underneath the facade. You already know things will not work out, so why string yourself along with someone you do not see a future with.
I definitely agree that your word is your bond, but she wasn't being completely real with you so its very irrelevant. No, it is not your fault. But just make sure to be careful with what you promise someone next time.
Most Helpful Guy
First don't be upset. Your probably wondering why a 17 year old has responded to this. I'm very weary of people in general but one thing I've learned if someone doesn't treat you right pr you feel that person is hurting you get them out of your life. In this case I wouldn't tell her anything I would slowly drift away from her and if she notices and asks why be honest and don't be ashamed. She found you to be Interesting enough, that's better than what some men get from women. I've never been in a relationship and your making me jealous :-) this is a lesson you should carry with you. You will find someone else and be upfront about what you want in the relationship and this time tell them you know when a woman is leading you on and you don't like it :-)