You wouldn't say it to a friend
Correction and attack
So far I've talked as if self-criticism were one thing. The distinction this book depends on is between two things that travel under the same name, and there's research behind it.
Paul Gilbert, a clinical psychologist at the University of Derby, and his colleagues asked 246 students about how and why they criticised themselves. When they analysed the answers, self-criticism came apart into separate strands. There were two forms. One was feeling inadequate and dwelling on mistakes. The other was harsher, a matter of self-disgust and wanting to hurt oneself. There were also two functions, or reasons. One was a wish to improve and to correct one's errors. The other was a wish to punish oneself, to take revenge on the self for failing. The researchers called them self-correction and self-persecution. It was the persecuting kind, along with self-hatred, that was most strongly tied to depression. People's capacity to reassure themselves worked against it.
That gives us the two things I've been calling correction and attack. They feel similar from the inside, because both arrive as a critical thought after something goes wrong. They differ in four ways, and once you know the four, you can tell which one you're hearing.
Correction is about the act, and attack is about you. "The figure on slide six was last quarter's" is about the act. "Stupid" is about you.
Correction is specific where attack is sweeping. "I didn't check the slides against the final spreadsheet" names one thing. "You always mess up" names everything and therefore nothing.
Correction looks forward and attack looks back. Correction ends in something to do, such as "check them against the final version next time". Attack ends in a verdict, and you can't act on a verdict.
And correction stops when the matter is dealt with, while attack carries on. Once you've rung the school, correction has nothing more to say. The commentary followed you across the car park.
Run the Wednesday through that. "Ring the school now" was correction, and it took ninety seconds. "Deal with school letters the evening they arrive" is correction too. Everything else was attack. "You idiot" contributed nothing to the fix, and neither did "what is wrong with you", "every other parent managed it" or the midnight review.
The discipline story depends on a confusion between the two. You've noticed, correctly, that a self-critical thought sometimes leads to your fixing something, and you've concluded that self-criticism works. But it was the correcting part that did the fixing. The attack came along with it and took the credit. They've arrived together for so long that you've never seen one without the other.
You can have one without the other, though. Plenty of people notice their mistakes promptly, feel a proper pang about them, fix them, learn something, and move on, without being called an idiot by anybody. You do this for Anna. When she told you about the dentist, you helped her think about rebooking, and you didn't assess her as a mother. You gave her correction without attack, so you already know how it's done.