Breathing is important and not just because you will fall over if you don’t breath.
We are trying to create a condition where the beat of your heart does not cause the position of the bow as you hold it to “pulse”. The two things that will result in this are energetic activity and holding breath in while taking the shot.
Everything you do on the shooting ground during practice or competition should be “measured”. Do not run, jog, leap around or even walk rapidly.
Give yourself plenty of time to get your equipment onto the shooting ground and prepared well before shooting is due to start. If you move bosses ensure that you have time to recover before shooting. At least 20 minutes. This way you keep your breathing rate and heartbeat steady and close to your rest rates.
As you approach the line to shoot you should deliberately take control of your breathing. Slow the rate at which you are breathing. In general terms if you slow your breathing rate you will also tend to reduce your heartbeat because the slow breathing rate is calming. This is the current advice for treating anyone having a panic attack. Slow the breathing rate which allows you to relax and the heart rate follows.
All your movements should be controlled and deliberate, also to slow your heartbeat, and keep you calm.
Breathe in to provide oxygen when you do something that will require the expenditure of energy. Hold the bow, aim and shoot with empty lungs. This minimises the transmission of the heartbeat to your bow.
You must work out a regimen that will suit you. It must be something that will become second nature, or at least a routine that you can follow with very little thought. As an example here is the routine that I use: