From a males prespective....in my opinion its "all in your head" so to speak. You think to much...you worry to much...you cannot relax and enjoy sex for what it is. Sex is not ONLY about the end point....its about the wonderful intimacy of getting to the end point (and the fun stuff). Start focusing on what your doing at the moment and don't worry about if you will "get there" or not. I would bet that you are focused on something other than the positive of what your doing when your having sex. Try to relax and focus on sex as ONLY a positive...I bet when you stop thinking so much and let yourself get absorbed in the awesome situation your in....you will be there. One moment at a time.
Do you feel true attraction to the man/men that you have been with?