Once you're friendzoned it's unlikely you'll find a way out.
In the past I've been interested in girls who saw me as their "gay best friend" even though I am straight and wanted to pound them. It's a sad position to be in. The more time I spent around them the worse it made me feel about it. Eventually we drifted apart and though we remained friends (at least with one of them), the less time I spent talking to them and generally being around them, the better I felt. Now a couple of years on, I still see one of the girls from time to time and if she fell in love with me I think I'd probably reject her.
I used to be the typical nice guy, cared too much about the girls I liked to treat them with anything but reverement, which is a big mistake. Nowadays, I focus less on girls and more on myself, I tend to act way more casual around them, and generally look less desperate to them. You don't have to turn into a dick, try to build your confidence and know that no girl is better than you. Be more assertive with things, show new girls you meet that you're self confident. They'll notice you.
The bottom line is you can be a good guy and still get women to like you, it's more about being confident along with not bending over backwards or becoming a doormat for them.
I recommend distancing yourself a from this girl. You don't necessarily want to give up on your friendship, but the closer you have to be to her the more it's gonna hurt that she continues to reject you. If you want to go hardcore then just go cold on her or cut her off completely. You never know, if you reintroduce yourself to her in the future when you're older and more confident she might realize what she missed out on.
Either way, the place you're in right now is a bad one, and you won't look back on it with happiness in the future.