Breathing deeper and exhaling slowly activates the diaphragm, which sends messages to the vagus nerve to turn on the parasympathetic nervous system. Once your body believes it’s in a more parasympathetic state (even if your mind is still trying to manage the stress), you’ll be calmer, more relaxed, and able to lower your heart rate.
This is vital for performance anxiety. Or any anxiety in general.
Inhale through your nose with your diaphragm (or your abdominal muscles), and gradually exhale. The more you’re able to train this diaphragm (a muscle that is essentially the lid on top of a cylindrical support for your spine, your posture will improve, your breath will be your anchor during times of stress, and your quality of life will improve.
Try it. Do the unusual.