There are many different suggested breathing techniques to help calm oneself down, but the one I have found to work best is 4-4-8; breathing in for 4 seconds, holding the breath for 4 seconds and exhaling for 8 seconds.
I use this before every test, when I can feel myself getting angry and any other time I start to feel myself lose control. It makes me slow down and focus on one thing. I count in my head, and focus only on counting. It momentarily gets me to put the issue on hold so I can focus on calming down. Then when I’m feeling calm, I can start again with a fresh outlook and perspective on whatever was troubling me. I wouldn’t call this procrastination a bad thing in this case, it’s a tool that is used for the purpose of later being productive. It improves my concentration, my mood and clarity of mind.
So if there’s something troubling you at the moment, try giving 4-4-8 a go. Let me know how it works for you, or if you have any other breathing rhythm suggestions.