If I understand, it is because of some abuse or traumatic experience with an ex, right?
I was molested as a child, and although I didn't have problems with the act of sex itself (after I was grown) there were certain parts of the foreplay that I had a problem with-basically, I could not touch a man's penis. And if they treid to 'guide' my hand, it wasn't unusual for me to freak...I finally learned to just keep telling myself that the person I was with was not the person who hurt me. This person cares about me and would never hurt me or force me to do something that I do not want to do.
Try doing that Sharlott, and see if it helps...I guess it really depends on what happened with your past relationship. I know there are some people who are so traumatized that they are NEVER intimate. At least you know that you can have a nice sexual realtionship/experience. You are lucky to have a man who understands and is willing to help you. Make sure you tell him that on a regular basis-he will appreciate your appreciation.
If you ever want to talk more in depth, just PM me, I would be glad to listen and see if I could help.