Be realistic and avoid frustration.
When you expect to perform perfectly or have a zero-mistake performance, you set yourself up for failure. Your goal needs to be reasonable and achievable. Thus, your first step to boosting composure is to come mentally prepared to competition knowing that athletes make mistakes and you have to accept this fact. If a score goal is not working for you (you get frustrated and disappointed) choose a different type of goal. Bogey or better on the hole, 2 putt the green. Or smooth strokes without force or tension. These will make you focus on each shot rather the whole round. You can even use an alternate scorecard so you keep track of these things instead. This will help you focus on goals for each hole.