"It's only in my head"

You might accept most of this and still feel it doesn't matter much. It's only thoughts. Nobody's hurt. You say a few harsh things to yourself and get on with the day. Compared with real problems, it hardly seems worth a book.

Consider how often it happens. If you kept the tally from chapter 1, you have a number. Suppose it was thirty remarks a day. Over a year, that's more than ten thousand, all from the same voice and all pointing the same way. If a colleague made thirty disparaging remarks to you every day, you wouldn't describe it as only words. You'd describe it as bullying, and you'd expect it to affect your health. It doesn't stop mattering because the colleague is inside your own head, where you can't get away from it by going home.

Consider too what repetition does. Anything rehearsed ten thousand times a year becomes automatic, quick and convincing. After enough repetition you no longer experience the commentary as an opinion. It seems like plain perception. "I'm useless at this" stops feeling like a thought you're having and starts feeling like a fact you're observing. It's how a habit of speech becomes a belief about who you are.

And consider what your body makes of it. This part is interpretation, based on a view that Gilbert and others have argued for, and I can't point you to a single study that settles it. The systems in your body that respond to threat don't distinguish sharply between an attack from outside and one from inside. If someone stood over you and said "you idiot, what is wrong with you", your heart would speed up and your stomach would tighten. In the car park on Wednesday, with nobody there but you, your heart was going. You were reacting to the form being late, and also to being shouted at. A person who's attacked thirty times a day, from inside, spends a good part of the day in a mild state of alarm. That's tiring, and it may be one reason why self-critical people so often describe themselves as worn out.

So the commentary isn't weightless. You hear more of it than of any other voice in your life, and it's always hostile.

You do have more control over it than over most things. You can't choose your colleagues or decide what strangers think of you. How you speak to yourself is one of the few parts of your daily environment that's entirely in your hands, though after thirty years of habit it doesn't feel that way. The next chapters deal with changing it.