Every “just this once” is how “every time” gets started.

The exception is where habits are actually formed. Each time you make one, you are not breaking a rule so much as teaching yourself that the rule has a door in it, and the door gets easier to find. The second exception feels smaller than the first, which is the whole mechanism.

Watch for the phrase in your own head this week and treat it as a signal rather than a decision. If you want an exception, plan it in advance, which is different from granting one in the moment. Rules you bend on purpose survive much better than rules you negotiate at the time.