Record what you say to yourself after a mistake. Would you let anyone talk to your little cousin like that?

The voice that arrives after a mistake is often harsher than anything you would tolerate from another person. It passes unexamined because it is familiar and because it sounds like standards rather than cruelty. Held up against how you would speak to someone you love, its real character becomes obvious quickly.

Write down what you actually said to yourself the last time something went wrong. Read it back as if it were addressed to someone small. Then write the version you would have said to them, and use that one next time, which will feel unnatural at first and then less so.