Our training revolves around us reacting and making decisions when that's happening. It unlocks your brain to move faster than it is used to moving because the faster you move and do what you are ordered to do, the sooner the yelling and chaos stops. It is so we don't become frozen in combat situations and make the best decision at that moment. The hardest part was the first 2 weeks when we started, those 2 weeks were when everybody was getting smoked for mistakes or freezing. Some learn it the first day, 99% has learned it by the end of those 2 weeks. 1% is Pvt Pile, and every platoon has 1 or 2, typically not more than 5.

My coach teaches this style breathing technique for active recovery to stop from being winded while in a fight:

Breathe in for 2 Hold for 2 Exhale and breathe 2 normal breaths Repeat

If you want to learn controlled breathing look into the Hindu practice of "pranayama". It is used as a control/discipline/meditation technique for thousands of years.

OTOH, if you want to achieve maximum oxygenation with each breath then I'd suggest start running/jogging. Your body will do what's natural and your VQ ratio (ventilation:perfusion) will massively improve.

BTW, stop smoking or vaping and you're golden.