A project manager needs to maintain his/her composure, particularly in tough situations and times. Being composed is a sign of maturity. It does not mean lack of concern. Though you may be troubled inside, loosing your composure in the presence of the team/stakeholders may weaken their performance and resilience to face tough situations. As an analogy (stated earlier): a project manager could be likened to a General in the army who faces a tough battle. Though he is troubled inside, he has to summon courage to act skilfully and inspiringly, without loosing his composure in order to motivate and strengthen the troops.
Maintaining your composure enhances your ability to make good decisions and operate effectively. To do this, you need to understand yourself and develop appropriate strategies to manage stress and stressful situations. Doing otherwise may cause more problem than you are trying to solve. You may be the last hope to straighten crooked situations in your project. Hence maintaining your composure is a great virtue.