What do you do when the love of your life does not love you back?

I am a guy in high school, please don't let that affect your answer, and I feel that I am in love with my best friend. Her and I have been best friends for a long time. She tells me I am the most important person in her life, and I believe her. I know she's the most important person in my life. Another way to put it - I once saw a music video of some people about to die in a plane crash, "At the bottom of everything" by Bright Eyes, and started to think about what I would do if I was about to die. I found the perfect way to say it in a song she gave me, "Cop Car" by Forever the Sickest Kids. The song said, "I should have called my mom, but instead I called you." I know that's what I would do, and she says that that is what she would do also. I love her with all of my heart, and she loves me. However, she doesn't love me as in a relationship love. We actually went out for a little bit, but then she said she couldn't see us as a couple. She knows I love her that way, also. I would do anything for her. I feel she is more important that life itself. I know she won't break up with her partner to be with me because she doesn't like me that way. I don't know what to do. I don't know how to deal with this heartache. I haven't gotten over her, and I don't know if I want to. I try to remember the great times we had when we were dating, and we are still having great times since we are still best friends. I just feel that the only way to show her how much I love her is by acting out my love, not just saying. I don't want to just have with her, I'm not like that. I just want to do anything I can to make her happy. I want her back, yet I know it won't make her happy, as far as I know, so I choose not to do anything irrational when we are together. Basically, I feel like I'm giving her up for her, if that makes any sense. Any guidance would be greatly appreciated and thank you for trying to understand and help me. In addition, I have never, ever thought about suicide, and I never plan on it.