I'm in love with my best friend!?

That's a hard situation. I would say wait a little bit longer. When you try to tell her maybe write a poem or have her listen to a song that says exactly how you feel. It may be easier then just coming right out and saying it. Then explain to her how you feel. That you have and always be there for her.... Basically exactly what you said above. She may realize that she feels the same. There is no guarantee that she will feel the same though. You may have to either risk the friendship to tell her or just remain her friend. I know its hard and not what you want to here but just let things fall into place things will work out