Breathe into a brown paper bag.

Plunge your face into a bowl of ice water for 30 seconds. It activates a diver's response, which is a lower heart rate and calm.

Deep breathing

Respectfully agree with ChristopherJackson - sometimes - Just - Stop !! Practice a Pause - Count to 10 - Breathe Deep with each number - seems silly - Yet - sometimes that helped me to Chill Out Or calm down -

Go to your Doctor