Most Helpful Guy
I had the same dilemma you're in with a long time friend of mine I knew from high school through college and beyond. I ended up telling her how I felt, but she always insisted she didn't feel the same way as I did. It didn't hurt our friendship at all.
I think had enough time gone by, you never know, but I moved out of state, she ended up marrying a really nice guy and now I"m happily married. In fact, we lost touch with one another, but not because of anything other than just time away and our lives getting in the way.
If you feel strongly about this, I think you should tell her, but also make sure she understands your friendship with her is very important, and she shouldn't feel any pressure in telling you the truth. If she doesn't feel the same as you, it'll sting a bit, but don't hold that against her and appreciate her honesty. I'm sure she probably already knows you must have feelings for her, as people can usually sense when another person has a higher interest beyond being just friends.
Good luck and don't worry.