You have different nervous systems in your body.
Your calm and regulated nervous system is called Ventral Vagal.
Your mobilised and immediate stress response system is your sympathetic nervous system
Your immobilised and overloaded stress or life threat response is your dorsal vagal system.
You can learn to connect with these systems using different tools of awareness and regulation.
The breath is a key to your body. Although structural alignment, nourishment and thought patterns, plus other factors are also vital to understand.