Aw, that's sweet
Well, if you sincerely have feelings for her, and you have known eachother for as long as you already have, I'm sure you could just tell her when you two have a moment to yourselves. Be honest with her. Girls love honesty. (Not sure if you are female yourself?) Either way, I always believe that honesty is the best policy. And instead of asking her to be your girlfriend, I'd say try to ask her 'How do you feel about that?' If she says she'd prefer to stay friends, then I guess you have your answer, and it's easier on both of you because you didn't put her on the spot to say yes or no to you right then and there.
It's a touchy situation, that's for sure, but the best way to approach this is to be honest, ask the question gently, and in an open-ended way, ie:
"I really like you, as more than a friend, but I wasn't sure how you'd feel about me telling you... what do you think?" or "How do you feel about that?"
If she says she really wants to just stay friends, then I would leave it at that and not pursue her as a girlfriend anymore. That could potentially ruin a relationship.
Keep in mind though, that if you two DO start dating and end up breaking up for whatever reason (Not saying that it WILL happen, but it's a possibility) Be prepared for you and her to potentially no longer be friends after that. Sometimes, when two people share an intimate relationship and end it quickly, it's near impossible to go back to the kind of relationship you two had BEFORE you were dating. Granted, this is not always the case, but it IS something to be made aware of.
Good luck!
