this is something my dad thought me. if you say you are going to mess up you will. go out on the pitch with a mean face. and try to go out thinking not about how you are going to screw up but thinking how can i dominate. also do not try to be perfect. if you try to be perfect you will also screw up: this maybe hard to believe but trust me on this. and never ever go out playing with expectations. why? because if something dose not go your way when you have expectations you get sort of a mental crash. you also sound scared. you can never be scared. if you are scared you will screw up. go and believe that you can do good. remember do not expect. i had the exact same experience as a goalie. then one day i went out leaving my worries behind and i was not worried only the other team was. i also herd one of the guys from the other team say fu** keeper won't let anything in.

Yes you have really mentioned very good and important things in your post as this will definetly increase the confidence of every reader.
Tia Smith
Weight Training