Hmmm there are several things that may have happened here:
1) You may have received mixed messages from her...ie. she was flirting with you because that's what people do.. and you took it too seriously.
2) She does believe you are attractive but she has just started a relationship she feels has a future...
3) She may not legitimately like you in the way you like her... she targeted you as friendship material and as non threatening...
Basically what you may end up regretting is not respecting that she has said no to a relationship with you right now...so:
What to do...give it some time... you have done the brave thing and already told her how you feel... so the only thing you can do is remain her friend for a while and see what happens...if it is meant to be it will happen...don't try to force a relationship on her... it won't work... it will seem desperate and girls don't want a desperate guy... act like you don't care about the rejection and she is more prone to wonder why...
Whatever you do don't start stalking her and telling her she is the only one for you and that kind of stuff...that will frighten the crap out of her... just go on with your life as usual... make other female friends and see how she pans out...like I said it may just be timing...maybe you will end up going out with her in a few years... you never know really...
I hope this helps
V.
BestVibes.ca