i am tired of this

i have been in love with my best friend for almost a year. She is so wonderful and amazing but i cannot be with her because she has a boyfriend. i wouldn't do anything that would break up two people who love each other. we are so close that we tell each other "i love you." a few days before christmas, she found out that he had cheated on her. i was there for her, as a friend, when she was crying and upset. to my knowledge, this isn't the first time. anyways, she took him back and it wasn't a week later he told her that he is not sure who he loves more, her or the girl that he cheated with. Again, crying and upset, she came to me. but she didn't break up with him and stayed with him. so a little over a month ago, she apparently talked to her sister about her situation and about me. She told me that she loved me and had a crush on me. so when i build enough courage to ask her out, she rejects me and tells me that she shouldn't have led me on and will never do it again. this has hurt me more than anything i have ever experienced. i was so close, and now they are supposedly "back in love" i may have lost my chance forever... this has just been killing me inside and just don't know what to do. i still want her in my life because she is my best friend so it is just making it that much harder to get over her.