Ssid,
As much as you're gonna hate this advice, it is the bottom-line truth. You got to let her go if you're not in love with her. If you sit and think on this one, it's kind of selfish to hold on to someone when you're not in love with that person.
You may feel you're not going to hurt her by continuing to date her, but the longer you let it go on for, the worse it's going to be for both you and her. It's just best to man up and tell her the truth.
And yes, she will be hurt by it, but if she's a strong individual, she'll get over it. She'll be hurt even more if she finds out she's in a relationship with you and the feelings are one-sided.