Tactical breathing (box breathing) is the standard arousal-control drill used by combat units, first responders, and elite athletes to keep the prefrontal cortex online under stress.
Channel the body's stress response so it sharpens you instead of freezing you.
Fear is not the enemy. Fear is the body loading its weapons. The enemy is fog - the freeze that happens when the load has nowhere to go.
Slow the breath and the body slows with it. Four seconds in through the nose, four holding, four out through the mouth, four holding. Two cycles is enough to bring the mind back.
Then fix your eyes on the answer, not the problem. Where is the exit? Where is your family? Where is the barrier? The eyes lead the body; the body leads the fight.