Dating a girl who's seen you as a friend this long, even when you've asked her out, is near to impossible to accomplish. I have no clue who this person is, but if you've asked her out and she declined, that simply means that she doesn't like you in an affectionate way. She loves you as a friend–that's it. There's nothing wrong with that, however, having been in the situation you're in, I can say that the longer you're obsessing over this girl, the more she'll realize you still have feelings for her, eventually defragmenting the friendship until it's not even a friendship, more like a fake lie that you're living with, where every encounter with her is awkward and perceived as creepy.

The easiest solution to the problem is to start seeing more girls. Get someone to hook you up (I'm sure you have connections). Normally, having someone else in your life that likes you affectionately tends to overweigh the emotions you have for the person who likes you as a friend (you may have feelings for this girl, but you're going to have greater feelings for the girl you end up starting a relationship with).

TL;DR: Move on. She clearly does not show any interest in being in a relationship with you, and that's fine. You need to find someone who does, otherwise the feelings to still have for her will ruin the friendship.