close your eyes and imagine a square, then breathe in and out as you imagine tracing the edges. see attached link for visual reference! very helpful breathing exercise for when you need to calm down.
My mom taught me this in order to slow down the nervous system. Breathe in for 4 seconds, hold for 4, breathe out for 4, hold for 4. If you're an anxious person you probably already know this. Also a good way to fall asleep really fast.