You've got to just keep playing, keep trying, and not give up.
The first tournament I played in was a pretty big one at JOB Billiards in Nashville. I was so nervous...I couldn't see straight, much less shoot straight (I think I was 16 at the time). It was really disheartening.
The funny thing was, the first player I drew was the best player in the tournament. In the first game he broke, rank 4 balls and hooked me behind the 7. I got up...studied the table like I knew what I was doing (please keep in mind a lot of people were sweating this match because of HIM, not me...LOL) and executed a perfect kick to make the 6. I pocketed the seven and due to my poor position play (which was common) hooked myself on the 8 (behind the 9 as they say). I carefully studied the table...beautifully kicked in the 8, leaving perfect position on the 9. Pocketed the 9...game 1 goes to me. The crowd thought I was some kind of champion for about 3 seconds. 20 minutes later, I lost the match 7-1. LOL. I lost my second match as well (just as nervous).
I didn't give up. Just kept practicing, playing, competing....and eventually you learn how to win. I swear a lot of it is self-confidence and once you feel like you're supposed to win...a great majority of the time you do.
Just step up to the table. Have a gameplan in your mind of how you want to run out the table (or the next few balls...depending on how many balls you think ahead) and try to not to think of anything else. Tell yourself that you've done it before and you KNOW you can do execute. A lot of it is just being confident and positive.
I know after that first tournament I never thought I would be able to beat anyone....but I stuck with it.
Try to look at every loss or poor day of play as a learning experience and see what you can to do improve.
Are you doing your pre-shot routine consistently in competition and practice?
Are you lining up properly everytime and not getting lazy on this?
Are you staying down after every shot and not jumping up?
These little things make ALL of the difference in championship play. Learning how to aim/pocket balls is the easy part. It is the little things that seperate the champions from the novice players. They do all of the right things, the majority of the time.
I hope this helps some. Pool can be so cruel somedays and so wonderful the next. I loved Colin's comments on the subject. He hit the nail on the head.