What goes wrong when you ignore that breaking bad habits requires replacing not just stopping?

The most common reason fails: Choosing a replacement routine that serves a different reward than the original habit. If your evening snacking habit is really about anxiety relief and you replace it with a healthy snack, you have changed the food but not addressed the anxiety — the replacement will not hold because the real reward was never the taste.

The fix: Pick one habit you want to change. Write down the cue (when and where it fires), the routine (what you currently do), and the reward (what craving it satisfies — be honest about the real reward, not the surface behavior). Now design three alternative routines that could respond to the same cue and deliver a comparable reward. Try each alternative for two days and note which one most fully satisfies the craving. That is your replacement candidate.

The underlying principle is straightforward: You cannot delete a habit — you can only replace the routine while keeping the cue and reward.