I will stay calm and clear my head when things get hard.

People trained for emergencies are not calmer than everyone else. They are running a list. Airway, breathing, circulation — in that order, out loud if necessary, because the list works whether or not the hands are shaking.

Calm under pressure is nearly always a procedure rather than a temperament. Sit down. Write the three things. Ring the one person who knows about this sort of problem. None of it requires the heart rate to cooperate first.

Which is worth having decided before you need it. In the moment there is no capacity to invent a method, only to follow one, and the ones that work are always dull and always short.