Why won't she be intimate with me?

G’day guys,

              I've met a girl who I have now known for about 1 year. I liked her as soon as I set my eyes on her, was and am so in love. At the time though my brother liked her (he doesn't now) so I didn't pursue anything, I ended up getting a girlfriend during this time, which is now over. So finally I got the courage up and I am chasing her so to speak, trouble is she knows my past, I've slept with quite a few girls of which I am not proud of and I feel this may be a factor (though not the main one) in the reason why she will not be intimate. In school she was quite a trouble maker, a bit of a rebel but now she is a lot quieter. Anyway she will kiss me sometimes, she will also tickle me a lot... sometimes she runs her fingertips on my back but she will not do anything sexual. She is not religous at all and she is 22. her longest relationship was 5 months, I ask her, why she won't be with me, and she always says I don't know or deflects the question... she never does anything first either... I will always have to kiss her... or if I say do you like me she will say "do you like me?"... I feel she is afraid of being intimate as she is 22... though I cannot figure out why, I have tried asking this question many times though I never get a real answer... does anyone know what may be up? if you need more info feel free to ask. Tom