The first step in overcoming this problem is to try to understand the fear and why you have it. Fears like these do not arise for no reason, and talking to a therapist would help you in this stage.

There are three main strands to treatment of fear of sex.

1. Understand what the fear is about and why it is there
2. Gradual exposure to the sources of the fear during therapy and homework exercises, possibly Sex Therapy, which is the most effective treatment.
3. Use of medication to help reduce the sensations of fear

You are likely to need the help of a therapist with this problem. it is not uncommon, and can be overcome if you keep going and recognise that there is nothing wrong with you. You just have an irrational response to sex. You have that response for very good reasons, and can change it if you wish. The really great thing about sexual response is that is can be changed.