First of all, let me say that I can empathize with your situation. When I was 14 I fell for my lesbian friend who was 18 and dating a girl I was also friends with. I had a really bad falling-out with my father, and this girl was the first person I felt I could really trust (except for my mother) after the situation occurred. At the time, she convinced me that all men were no better than my father, and that lesbianism was the only satisfying solution to my trust issues regarding men.
To make a long story short, I thought I was in love with her for a number of years--at least 5. She knew how I felt, and she said she felt the same way, but used me more than once during those 5 years--though, thinking back on all of her relationships, she uses everyone. Finally, I got out of the "friendship," reevaluated my life, and came to the realization that I am not a lesbian and that this girl manipulated and used me when I was at my most vulnerable.
I am relating this to you, because I think you should get out of this friendship. I think you're just going to get your heart broken. I'm sure this girl isn't stupid--one usually knows when someone likes them in more than a "friendly" way. Does she date? I'm assuming you haven't dated anyone since the two of you have been friends? Knowing how she feels about homosexual relationships, if you tell her how you feel: prepare for all of her friendly love for you to turn into morbid disgust. Of course, this is the worst case scenario, but you ought to prepare for it.
If, however, she says that she's not romantically interested in you, but that she's fine with you being a lesbian: be weary. It is possible that she will just use your feelings for her against you, ultimately emotionally using you.
Now, again, I am just sketching out worse-case scenarios that you ought to be prepared for. It is possible that you'll tell her, that she'll feel the same way, and that the two of you will live happily ever after
. If she's truly your friend, but doesn't feel the same way: she will accept you for who you are and remain your friend. Just know that remaining friends with her, whether you tell her or not, will be extremely difficult for you--friendships with unrequited love are not often happy ones.
Only you know the depth of your friendship and your friend well enough to make an educated-guess as to how she'll respond. Just evaluate the situation, don't act rashly, and do whatever you feel is best. Be honest with yourself and try not to act based upon romantic delusions of grandeur.
Good luck and let us know what you decide to do!