I can only orgasm one way: with my legs straight

Jacki
 I have always cum this way. My legs have to be straight (either me on my belly or on my back), and my legs and butt all clenched and toes pointed. I can't see to come any other way. This sucks because I can't really be in such a position when I'm having partner sex, so I have never actually cum by penetrative sex. I have only cum by clitorial stimulation and clenching of the whole lower body, legs straight. Sometimes when having sex with a partner I feel like I'm so close to cumming but I never do, and I feel quite disappointed when I can't release after all the build up. What should I do to 'train' myself to cum another way?