Are some women simply unable to get an orgasm?

I am 19 years old, I have had 7 partners who have been all different sizes and age. Varied from a 17 year old to my current boyfriend who is 27. I have never had an orgasm though, not even by myself.:(. i've had oral sex, stimulation of the clit etc.. I still get pleasure out of having sex, more so when I'm on top but i know i've never had an orgasm which not only frustrates me but my boyfriend who has been trying. We have been doing "doggy style" lately which feels even better but I'm still no where near getting one.