I think with a problem like this, you need to visualize.
Close your eyes, and think back to being a child of eight. Your mom and dad are fighting, and they might get divorced and its your fault. In the closet, under the bed, and everywhere you can't see, there are scary, scary clowns, with red, staring eyes, and sharp, bloody teeth. You are going to play bass, and your third grade teacher, who hates you, is watching, just waiting for you to make a mistake, and rap your knuckles with a ruler. Every woman who ever rejected or ignored you is also there, and if you can get through it all with perfection, they will all sleep with you, and profess their undying love. The Russians and the Chinese are there too, and if you get even one note wrong, they are going to start WWIII, and kill lots of babies, just for the fun of it, and make their parents watch. Oh, and torture puppies. LOTS of puppies. And kitties too.
Note that this is parody, and not an actual exercise, and smile. It's just an extreme example, to make you think of just how terrifying you have made something that is fun, and is actually of little consequence. If you need to get some help, please do. Don't let the fear win. Just play something, anything, and try not to worry.