You can use a breathing exercise anywhere, and especially to calm
yourself when under strain.
This breathing exercise consists of three parts: breathing in,
breathing out, and resting.
1. Breathe in deeply through your nose, right down into your stomach
and use your diaphragm (not just your chest). At the same time,
be conscious of the fact that you are breathing in energy (oxygen).
2. Passive exhalation: As soon as you have breathed in to your
limit, let the air slip out (like a kind of "sigh").
At the same time, notice the relaxation in your body, and let this
spread from your shoulders, right down into your arms and legs.
3. Rest: Continue the relaxation for a few seconds, for as long
as you wish, before breathing normally again. At the same time,
direct your attention towards a sensation or an image in your mind's
eye: you can look at something particular, or you can close your
eyes and imagine, for example, a flower. You might also choose
to listen to the sounds around you.