It sounds like you have stronger feelings for her than she has for you. Read this article:
Poorly-set precedent: it's the scourge of relationships across the face of mankind. Every day, the whole of the male sex collectively writhes in agony at its own terribly-set precedent coming back to haunt it - and bit it right in the ass.
www.girlschase.com
Men chase sex, women chase relationships. My advice is keep sleeping with her, but try not to get your feelings wrapped around in it. Is she the only woman you are having sex with? Since she does not want a relationship, see if she will stick around for a friends with benefits situation. And while you do that, start sleeping with other women.
I dont know much about your background but if you keep swinging at it, Im sure you will find a woman who wants to be your girlfriend. Not every guy wants to keep playing the game and that's 100% ok. Just ensure you are ready for a relationship though.
Troy