If you want to calm yourself down when you are feeling overwhelmed and stressed a first step is to be able to control your breathing. "Box Breathing" is a Navy Seal breathing technique and helps one relax by building a bridge between concentration and awareness.
People with high-stress jobs, such as soldiers and police officers, often use box breathing when their bodies are in fight-or-flight mode. This technique is also relevant for anyone interested in re-centering themselves or improving their concentration.
The four part method is as follows;
Expel all the air out of your lungs. Keep your lungs empty for four seconds.
Inhale through your nose for a four count.
Hold. Keep the air in your lungs for four seconds (Do not inhale or exhale).
Exhale smoothly through your nose for four seconds.
Repeat for at least five minutes. Do this anytime you need to calm down. The 4-4-4-4 count is a box, hence the name of this technique.
I found the following app quite useful in perfecting the technique.
Box breathing: How to do it, benefits, and tips. https://www.medicalnewstoday.com/articles/321805