How to stop catastrophizing about small mistakes?

Don't try to just "think positive" and force yourself to ignore the worry - that usually backfires and makes you fixate more. Also avoid spiraling into "what if" scenarios by yourself; the more you replay the mistake alone, the bigger it grows in your head.

The actual fix is to reality-check what you're catastrophizing about. When you catch yourself doing it, ask: what's the worst that actually happens? Not in your head, but in reality. Most small mistakes - a typo in an email, forgetting to mention something in a conversation, a minor error at work - get forgotten by everyone else within hours or days. Write down what you think will happen, then write down what realistically will happen. You'll see the gap pretty quickly.

Also helpful is to talk about it with someone. Not to complain endlessly, but just say it out loud to another person who can reality-check you. They'll usually tell you it's no big deal, and sometimes just hearing another human say "yeah, that happens to everyone" takes the edge off the panic. Finally, do something that requires your full attention afterward - work on something engaging, go for a walk, whatever keeps your brain from chewing on the mistake. The catastrophizing usually feeds on boredom and free mental space, so occupy yourself and you'll move on faster.