Just don't think about it. Tell your folks or the doctor to avoid telling you until you're there. Out of sight out of mind. Let it go, it doesn't exist so don't worry. When you get there-you need a shot--bing bang boom It's over.
I'm with you. I just tell whoever's administering the shot that I really don't like needles, but I know I'm just being a wimp about it. They proceed to reassure me that lots of people have problems with needles & shots, then they either distract my attention from the needle poke and/or they're really gentle with it. I still don't like it, but everyone seems to try hard to make it as unpleasant as possible.