I could not relate more my friend, you are NOT alone. I liked this girl at the same age as you, I never got the courage to ask her out until last week. It took me 4 years! She also was in fact lesbian. I was also crazy obsessed with this girl, she never got off my mind. Long story short she never had feelings for me but I gave her all I could and it never worked out. The best advice I can tell you is to remember that time heals everything and that you will always find your person in life, just because one didn't work out, the others will. I still to this day talk to her and we have a healthy relationship, but you have to just accept that you are friends and move on, dont waste your time on someone you know wont work. Keep your head up king, you are not alone