Honestly, getting over a girl is never easy.
I'll tell you now: she isn't the girl of your dreams. If she was, she wouldn't be fucking with you the way she is. I understand that you "love" her. I know exactly how it feels to think that you're in love. The reality of it is, love is not wanting nothing more than someone to be yours, it is wanting nothing more than to be someone's. Teenagers (oh and, I'm sixteen; I'm a teenager myself) often tend to feel as if the world is over when they lose someone they think they love. Then, lo and behold, someone else sweeps them off their feet and the cycle begins again.
My suggestion to you, OP, is to simply direct your attention to other things. Take some time to learn how to love yourself - go out and do your favorite things. The only real way to ease the pain of a broken heart is to find someone new. I know it sounds difficult, but open your eyes to other people! There are bigger and better things than her. There are 3,386,509,865 (estimate) girls in the world. You'll have plenty of different relationships as you age (you're only fifteen). You'll find plenty of other girls - just take this opportunity, now that you're single, to flirt! Have fun with your life.
Just take the time to hang out with friends. You can have girls who are just friends - you don't need to be committed to one person. You can have fun (sexual pleasure, too!) with just a friend with benefits.
Oh and, believe me, once you meet the person you were meant to be with, you'll understand why no one else ever worked. You'll understand why this girl likes your best friend instead of you. If she truly is the one for you, you'll want nothing but her to be happy, and if your best friend makes her happy, all you can do is be happy for her.