Practice yourself, for heavens sake in little things, and then proceed to greater.

Big tests of character or skill arrive rarely and without warning, so there is no rehearsing for them directly. What prepares you is the small daily version: staying calm when a queue moves slowly, keeping a minor promise, finishing a short task properly. Those moments are low stakes, which makes them the safest place to build the habit before it is needed. Waiting to be steady until something important is at stake almost never works.

Pick one small irritation you meet nearly every day and decide in advance how you want to handle it. Practise that response for two weeks. When something larger comes, you will have somewhere to stand.