I don't know why other posters are saying she's using you. It doesn't sound like she's stringing you along at all. You expressed your interest, sounds like she has politely and consistently told you 'no'.
Searock has the best advice IMO. Stay friends since you enjoy it (why not?) but look for romance elsewhere. If she changes her mind, she knows where to find you. If you are dating someone else by then, too bad, such is life. Besides, good friends are much harder to come by than BF/GF.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh