I inhale confidence and exhale fear.

The out-breath is doing the real work. Breathing in speeds the heart slightly; breathing out slows it. Make the exhale longer than the inhale and the body begins standing down, whether or not anything in your situation has changed.

Singers know this. So does anyone who has waited in a corridor to be called into a room.

It's a small piece of machinery and it is genuinely yours — close to the only part of the nervous system you can operate by hand. Four slow breaths before a difficult thing won't remove the fear. It lowers the volume enough to hear yourself think.