Should I tell a girl I love her?

She has a boyfriend that is a good friend of mine and a good guy, she and I share everything in common from a love of airplanes and flying, to English horseback riding, and even sailing. It seems like we would be the perfect couple, but she's three years steady with this guy and she's gonna go to college at Central ******* Washington (not a prestigious school) with him (when she could go anywhere she wanted to in the world) and probably marry him. Well, for three years now I've always felt differently about her than other girls. I don't want to pretend I know what love is, or that I'm somehow unique, so I'm asking you guys what should I do. If anything, I just want the best for her, and if that means disappearing from her life after confessing my love, I can do that. I can't stand to be around the two of them together, but I can't stand to be away from her.