When you've started another one
It'll happen. A bad fortnight, a photo, a remark, a January, and you'll find yourself on day three of a plan with an app on your phone, and you'll realise, and the thought will come that the book didn't take.
It took. What happened is that the cycle has run for years and it ran once more, and you noticed on day three, which is earlier than you've ever noticed before. The noticing is the change. Last year day three was a good day on a diet. This year it's the moment you see the envelope.
Stop the plan, not yourself. Delete the app, or close the notebook, and have lunch. Not a special lunch, not a last supper, not a smaller one to be safe. The lunch you'd feed a friend. The diet ends now, in the middle of a Wednesday, with no write-off and no Monday, and that's a new kind of ending for it. Every diet on the envelope ended with a break and a blame. This one ends with a sandwich.
And notice what you did on day one and two. You probably had a last supper on the Sunday, and you probably ate less than you wanted on the Monday, and you probably felt in charge, and all of that is the cycle doing exactly what chapter 4 said it would, on schedule. Seeing it run, from inside, and recognising each part as it arrives, is worth more than any number of chapters, because now you've watched it, and a thing you've watched is a thing you can stop.
Work out what set it off. A number on the scales? Then the scales go away for a month. A remark? Then chapter 15 is the one to reread. A fortnight of uneven eating that felt like a flood? Then chapter 13, and the reminder that the flood follows restriction, and you'd been restricting.
No streaks. Don't count diet-free months as a run that a fresh start breaks. Count meals that were meals, and let day three of a plan be three meals in a long row that were mostly ordinary. The envelope gets one more line, and the line says "ended Wednesday lunchtime, with lunch", and that's a different envelope from the one you started with.