[Splat]

I guess I can offer a little explanation, what with studying psychology.

Repression is one of the mind's natural defences. Basically your mind is hiding away the memories of what happened to protect you. Sigmund Freud (the first realy psychologist, who had a lot of good ideas and a lot of crazy ideas) would have said your mind is hiding the memories to protect your from psychological harm. Being treated in that way as a child probably wouldn't do you any good, and though you would probably be able to cope with that memories now they're still repressed. At the same time I think your mind has learnt to associate experiences like lying on your back and being touched, with experiences of fear or pain. So when you lie on your back your mind thinks "Oh no! When this happened before I got afraid, so I don't want to do it now!"
I guess most of that if fairly obvious, but both things are treatable. I know it can be tough, and I don't know if I would have the courage to do it myself, but you could talk to your doctor about it. Then you might be able to get help for the main problem. It doesn't mean you're crazy or anything, it's something that could happen to anyone.

Whatever you decide to do I wish you all the best. I'm praying for you.