The biggest reason I have seen for what you describe is that people run 5 or 6 balls pretty well then see that they have a chance to run out and try to cinch balls instead of playing the same way they were before. You end up miss hitting a ball and getting out of line a bit then the next shot is even harder and you either miss a ball or end up with a shot that is beyond your ability. To play well you have to practice, for me it's not the tough shots that beat a person, it's the easy shots they take for granted and don't bear down on. I would recommend 1 of the Bert Kinnister tapes
I believe his material is great for the majority of players. If you play with the intensity he tries to instill and put in the practice time necessary to play with confidence, your game will improve greatly.