Why am I not able to enjoy sex?

Let me start by saying that I absolutely love and adore my husband. We have been together for 4 years now and I cant imagine being with anyone else. I have one problem though. I cringe whenever he initiates sex. Sex is something I have never enjoyed, never had an orgasm or even come close! If he touches me in a certain way I immediately try to change the subject or distract him. I wish I could enjoy the activity but it always goes south as soon as I think of what's to come(no pun intended) I need tips and a little assurance that I am not the only one who avoids sex at all costs!