I combine a kind of modified "rectangle breathing" with meditating to calm myself.

I pause for a two-count while thinking "I am"...
Then to a four count while inhaling or exhaling, I think (for example) "Safe, calm, at peace, strong"...
Two-count pause, repeating "I am"...
Then to a four count, repeating my earlier meditation as I exhale or inhale.

It works for me!