Just tell her that you don't like her in that way, but you would still want to be friends. She might get mad, but most likely she will be okay!
I was asked out by this girl. She LOVES me, head over heels. I don't love her though. We're dating now. But I have a crush on someone else. Help?
So she is your sympathy girlfriend? You are with her because no one else was available at the time? I suggest you tell her you want to break up because you need to go down a different path at this point and don't see the relationship with her as part of your journey right now.