How do I stay calm during confrontations at work?

You have the job down. What still shakes you is the human part: the person who argues, the exception that does not fit the rule. That is not a gap in your competence. Confrontation stirs the body whether you want it to or not, and the exceptions rattle because they ask you to decide alone. Remember what your calm is for. You are not there to win or to be liked in the moment. You are there to hold a line steadily so that everyone stays safe. When someone pushes, they are testing the line, not you. Their heat does not have to become yours. Next time, slow your first response by one breath before you speak. Keep your voice low and your words few, and repeat the rule plainly. The pause is small, but it puts you back in charge of yourself, which is the only place control ever starts.