Okay... Your situation sounds really familiar to me. If you can bear with me long enough to get through my story, maybe I can give you some decent advice.
I had a thing for this girl back in high school. We'd been friends since kindergarten, but we got really close in freshman year. She was nothing like other girls; she was down-to-earth, smart, quiet, funny, a bit naive, not conventionally pretty (but still beautiful to me). I fell for her, and hard.
I pretty much kept my feelings to myself for most of my freshman year, and it drove me nuts. One night, I was talking to a friend of mine about the whole situation, and she suggested that I just ask this girl out. Well, okay, what she actually said was "You're a gutless loser. Just grow a pair and ask her out." and she hung up on me. So, I called my crush up, and the conversation was as follows:
"What would you say if I asked you out?"
"I don't know."
"Well, let's find out. Will you go out with me?"
"Uh, no, I think we're really good friends."
"Okay. Cool." and I hung up.
We didn't speak to each other for a year after that.
So, toward the end of sophomore year, we started getting close again, and that was cool. We were in a lot of the same classes, and worked on a lot of projects together, letting me spend a lot of time with her in and out of school. Sometime during that year, I took her out to the movies (the American Godzilla, sadly. It was the only thing playing at the one theater in my little town.), but things stayed friendly.
I was feeling a little bit brave after our "date," so I got a friend to ask her in conversation if she'd ever consider going out with me. Her response, burned forever into my mind, was "I've told him no a thousand times. When will he get it?" I don't know if she ever knew that I asked our friend to ask that question, or if she knew that he told me. Needless to say, I was no less in love, so I kept spending as much time with her as I could, and things continued pretty much the same.
Skip forward a bit to junior year. The prom was coming up, and I wasn't even going to go, since I knew I didn't have a shot with my crush. Hell, I'd even tried to help set a mutual friend up with her, since she seemed to like him, but apparently I was wrong about that.
One night, the phone rang. I picked up, and it was my crush. I figured she was calling to ask what that night's Spanish homework was. Well, she asked if I had a date to the prom. I told her no, and she asked if I'd like to go with her. It took me about two seconds to answer, mostly because my brain replayed the question about a million times to make sure that I heard correctly. Obviously, I said yes.
That prom still stands out as one of the best nights of my life. I looked handsome, she looked beautiful. The prom was at a nice country club. The food was good. Most importantly, though, I got to slow dance with her. Several times. Turns out that neither of us had slow danced with a non-relative before.
Let's skip forward again a bit. Senior year went well. We stayed close. We hung out a bit during the summer after graduation. Then, we just sorta drifted apart. We saw each other in town every once in a while, but the conversations were strained and awkward.
I just got in touch with her again about a year ago (I graduated in 2001, by the way), since I wanted to apologize for chasing her around like a lovesick puppy and wanted to know how my friend was doing. I got a very polite- curt, as well- response that she didn't remember me acting like that, and that she never felt any sort of awkwardness. However, she didn't feel the desire to stay in touch, since our lives took us in completely different directions. She briefly mentioned her fiance and her grad studies, and said that if we ever crossed paths again, at least we could probably have a conversation without the awkwardness. Given how much has changed, that's an astronomical "if."
And that's the end of the story.
I spent four years of my life chasing this girl and missing out on so many other opportunities. I spent a few years beyond that pining for her, leaving me unable to fully commit to a real relationship.
Thankfully, I've reached a point now where I can understand all of it for what it was. I'm engaged to a wonderful woman, and she's going to be my wife two years from now.
So, here's my advice to you: If she doesn't want to go out with you, it won't happen. I won't say that you're wasting your time, since her friendship is still something special that you should try to hold on to, but you need to accept that this girl doesn't want your heart. No matter how different she is and no matter how much time you put into it and no matter how great of a guy you are, it won't change anything. You're her friend. If you keep pushing things, you probably won't even be that. Cherish her friendship for what it is, and try to find someone else you like that might actually return the feelings.