I tell my students to not spend any energy worrying about things you cannot control on the athletic field, court, or course. By worrying, you waste energy on irrelevant factors beyond your control. As a result you become upset or frustrated, lose your composure, and it affects you for several minutes to follow or even until the end of the competition.
For improved composure, you must learn to train yourself to focus only what is under your direct control in sports. You can control your thoughts, behaviors, performance, and reactions in both practice and competition. Learn to distinguish between things you can control and things you cannot. This will decrease your worry or frustration, and improve your composure in sports.
Check out The Composed Athlete program to improve your composure.