What should I do about this?

I'm a girl. I've just got out of an 11 month relationship, 6 or so months of long distance. For the past month or two I've found myself falling for another girl, but she's a total 'head feck' and enjoys messing me about.

Basically, the girl I like has a boyfriend at the moment. She also claims to be 'straight', which I respect although I don't believe it. And to cut a long story short, she's leading me on, flirting and using me. The problem for me is that I am - not obsessed, but really falling for her, to the point where it really gets me down. And it won't go out of my head, month after month. She's using me and basically torturing me with it and won't stop. She's always touching my hair or messing it about, kissing my cheek, hugging me, or making suggestive and sexual gestures or actions as a joke. She also knows fine well she can get whatever she wants from me by doing something like that.

I've never really told her why it is I like her so much, she assumes it is because I think she is attractive. Of course this is part of it, but my attraction is more of an attention seeking, cute sort of thing, but incredibly strong. It's so hard to explain, like I love her deeply as a friend, but I know in my brain it's more than that I feel.

i don't make an effort to tell her to stop, and I just let her get what she wants and carry on. My problem is that I will admit I seek attention from her and there is a constant longing that won't leave. It's got to the point where I sit up at night and drive myself insane over it. Everyone else can see the hopelessness of the situation - I am falling hard and fast for her, she leads me on and uses me, and I'm not making an effort to stop it, if not feeding off the little attention she gives me. She is giving me false hope, because in my own head I realise I have no chance. However I carry on to let her do it, and don't explain how much it annoys me.

what on earth should I do? thanks all.