When you held again
It'll happen. Some Tuesday three weeks in, the turn will come, and you'll notice it, and you'll say the roadworks are nearly finished anyway. You'll know while you're saying it. And afterwards there'll be a particular flatness, worse than the old kind, because now you know what you did.
Two things about that evening. The first is that it doesn't undo the three weeks. Lally's study, from chapter 23, found that missing a day of a new habit didn't much affect whether it formed. A hold is a missed day. I'm extending that from the daily routines they studied to this, so take it as my reading, but the shape fits what I've seen. The habit is the noticing and the sentence, and you noticed. What you didn't do was the second part, once. That's not the habit gone. That's the habit with a gap in it.
The second is that the climb-down is still available, and late is fine. The rule says the same conversation, and that's the aim, but the next conversation is the fallback and it's a good one. "About the roundabout on Tuesday. I remembered the roadworks halfway through and kept going anyway. You were right." That's four sentences and it does more than the five words would have done on the day, because it also admits the holding, and the other person has almost certainly never heard anyone admit that. It's the strongest version of the climb-down there is.
Then ask which belief was doing the holding. Go back to the chapter titles. Was it the audience, chapter 4? The gloat, chapter 17? The sense that you'd said it too firmly to take back, chapter 6? Usually one of them was louder than the others on that day, and knowing which tells you where the practice goes. If it was the audience, the next few climb-downs want to be in front of people. If it was the gloat, practise the silence.
What doesn't help is going over the evening. The earlier book It's already over is about replaying old mistakes as if another viewing would change the ending, and the held argument is exactly the kind of scene that invites it. Do the late climb-down, note the belief, and let the scene go. There'll be another turn by Thursday, and that one's the one you can still do something about.