just one more
Allowing for "one more chapter, bite, etc." is essentially training the decay of self-imposed boundaries. Every time you do that, you are teaching yourself that the limits you've imposed can be broken, leading to cycles that build unintended momentum in directions you shouldn't go. Sadly, you will then need even more willpower to get back on track.
Thinking back to what varies and what stays, these are like the small habits that keep a system in check, but harmful ones this time. They build progress toward unintended goals, such as becoming greedier, unhappier, and less tolerant.