I have accidentally came up to an article by vice just recently and started doing the exercises in the morning and before sleep, sometimes. Was kinda surprised that it really works well and calms me down when needed, the technique itself is somewhat straightforward, you have to take 6 seconds to inhale and another 6 second to exhale, which makes for 5 breaths per minute. The article is called "This Breathing Exercise Can Calm You Down in a Few Minutes" if you want to look it up.
I know that method. There are a dozen variations of it. The more you use it the better. :-)
Have you tried to breathe underwater? It is very good, it relaxes and if you are exercising, swimming or whatever,
when doing the breaths you are directly bombarding your lungs with another type of necessary oxygenation,
which is to suck air through the nose and throw it gently under the water mouth.
Send me a video of you doing it and I'll might try it as well
