Screwing up on the course has a lot to do with thinking too much about your final score or finish in a tournament. When you do that, you can create unnecessary pressure that can throw you out of rhythm and can destroy your round. If you find that happens to you, do your best to forget about the result.
Stay focused on the shot you are about to hit. Forget about the last shot; don't think forward about the shot after next. Box yourself in with focus on only the shot you are about to hit. Don't let anything else enter your mind other than the visualization of ball flight and landing spot. Stay in that zone by guarding your thinking. Maintain this approach through the round; the results will naturally follow.
Like the old saying, "If you take care of the little things, the big things will take care of themselves." This is an example of that while you are playing your round. Practice this, and the results will come.