"Stopping would feel rude"
Now the practical fear. You know how the sorry comes out. You come through the doorway, someone walks into you, and the word is out before you've thought. Suppose you catch it. What comes out instead? Silence? A look? The silence feels rude, and the rude silence is worse than the reflex.
Two things. The first is that there's a pause, and the pause is short, and it fills. When you catch the sorry in the doorway, what's left is about half a second, and half a second in a doorway is nothing. The other person, who walked into you, says sorry, because they were at fault and most people apologise when they are, and you say "no problem", and the doorway is over. What felt like it would be a rude silence was the space in which the other person's sorry arrived, and your reflex had been filling it for years so that they never had to.
The second is that the pause has sentences for it, and chapter 21 lists them. "Excuse me" for the doorway. "Quick one" for the question. "Thanks for waiting" for the lateness. "After you" for the queue. None of these is rude and all of them are what most people say, and the reason they feel rude to you is that you've been saying sorry in their place for so long that anything else sounds like an omission. It isn't. It's the ordinary sentence, and the sorry was the odd one.
The flinch is physical, and it's worth knowing that. When you catch a sorry, there's a small bodily lurch, a sense of having stepped off a kerb that wasn't there. That's the reflex being interrupted, and it feels like a mistake because the reflex is old and the interruption is new. It isn't a mistake. It's the half-second, felt from inside, and it gets shorter every time it's allowed to happen.
Expect the first week to feel strange. You'll catch a sorry, leave the half-second, and feel that something's been left undone. Nothing has. The other person didn't notice. In my observation the strangeness lasts about ten days and then the replacement sentences arrive on their own, and the doorway becomes a doorway.
There's one place where the pause is genuinely uncomfortable, and it's when you've caught a sorry that was owed. You were the one not looking. Then say it, once, and mean it, and that's chapter 22. Stopping the reflex doesn't mean stopping the real one. It means the real one gets the half-second, instead of losing it to a word that was already out.