Girl I like doesn't like me back?

Ok...this girl I like, I've been unsure if she likes me back. I've seen her playing with her hair and looking at me soetimes when I'm not looking. I do things like compliment her and post things on her fb wall to show I like her even more. And she posts similar things back. But I just found out she doesn't Like me back. I get that you cant controll who you like and I respect that. But wtf? It's not really fair to me to flirt back If she doesn't like me back. I think she likes my best friend and I know he likes her. But just cuz of her looks. He's already stole the girl I Like once and ruined her life. I'm very lost and confused and sad and pissed off. We also hang out and play manhunt sometimes.. And another thing, she had a picture that said if ur a guy and tagged ur my friend, I wasn't tagged but my friends were. Help me please?