Make a deal with God before you drop in.
But seriously. Get used to the jump first, if you haven't already. Then try to stop freaking yourself out. Concentrate on the basics like arching your back trying to spot your landing as fast as you can. Don't try them when you are tired, cause fatigue is the killer.
You'll be fine. I too have a habit of imagining the absolute worst possible scenario in every situation. Try to block those thoughts out, and never ever hesitate. Don't take 15 minutes at the top contemplating the shit out it either, cause that's the worst thing you could do.