Lots of people have issues with how they look and their low self
image makes them acutely embaressed at the thought of having sex.
People who have been repressed also find it scary to enjoy physical
pleasure in front of another person.
If you have a real physical deficiency that is the cause of
your discomfort, speak to your doctor.
Start wearing more revealing clothes in private in front of the
other person till over time you feel more comfortable in your
own skin in their presence.
Petting, hugging, kissing, cuddling, massage without it
ending in sex may also releave some of the fear that you have.
You may turn off the lights while having sex till you feel
more comfortable.
Sometimes alcohol can lower your inhibitions and the level of
fear. Take a big gulp just before sex but don't make it a habit
because alcohol can become an addition.
Talk it over with your partner, they'll usually be
accomodating.
Fear of sex is not as uncommon as you think.