Girls: I'm falling in love with my best frnd, knowing she's already commited. help me make a decision pls.?

it all started wid a fight outta possesiveness, when she bcame my best frnd. But...gradually her excess affection on me, makes me fall for her, worser still,,,,knowing that she is commited with another guy. I try to get away frm her, but she just wud'nt let me. Wat shud i do now? Its such a pain loving sumone, knowing she cud never be mine.help me frnz.