I'm crazy about someone who doesn't love me back?

I'm a girl, and I'm in love with another girl, who just happens to love another girl. Complicated, isn't it? I know I'm only in high school, but I'm in love. Don't tell me I'm 'too young', because I believe you can fall in love at any age. You don't have to be mature or 'grown up' to know what love is. And in my opinion, I know what it is. I have told her that I 'liked' her, but I didn't want to freak her by saying I love you more than my heart's contents, and I'd do anything to be with you. In my eyes, she is perfection. And I know its not just some thing, that I'll just get over, like she's not going to get over this other person. I know its never going to happen between us, but I can never stop adoring this girl. And i can't just block her out, with have classes together and I see her around school a lot. We don't talk often, so I can ignore her physically, but my brain won't ignore her. I'm so nervous around her that I can't even look at her when we walk past each other. Sometimes when we accidentally look at each other, I give her a weak-looking smile, its the best i can do. How can I stop thinking about her all the time?