Hello,
I think that you should try to relax and not to be ashamed to express yourself, your feelings and needs! I was having some problems in the past too because I couldn't relax and I wasn't feeling comfortable expressing any kind of feelings, I was feeling some kind of shame and I was embarrassed...
The mind is the main problem and how you see on the sex/making love.
In my case, I solved the problem when I understood the real meaning of sex/making love. It's also important to do it with someone you trust, because in that way you can let yourself relax completely and enjoy, feel comfortable. Also, sex and making love are two different things and you can feel so much more and experience so much more and on a different way with someone you love (and he loves you) and someone you are close to and there is mutual trust. With someone like that you feel more secure and you can relax more and enjoy and feel good.
But, the most important thing is how you see on sex/making love and how you see yourself in that. Maybe it's too much for you because you are so considerate and you don't want your partner to wait for you, so you don't feel comfortable and you stop, it's too much pressure and effort. But, remember, you are also a part of it and it's natural that you have needs and you are also important. So, think about your partner, but also about yourself, because that's the only way you can feel good and satisfied and if you are feeling good, the partner that cares for you will feel the same way too.
You might be having a block to the actual letting go and allowing your body to enjoy an orgasm. Think about your attitude to sex, did you get told it was dirty, or something only men can enjoy? Were you raised in a very strict family? There are lots of reasons women fail to enjoy an orgasm and you really need to relax and think about your own feelings. It is not that you aren't capable, it is your mind that is stopping you. Have a think and maybe write it down and I am sure you will find the answer and then you can deal with that and enjoy a happy sex life.