Exception = the breaking of a constraint.
Exceptions are necessary. Consistency is crucially important, but stubborn rigidity can have some negative effects when it’s held for too long. Small rule-breaks will eventually happen - but they don’t have to be highly damaging.
There are two main ways to make sure that the exceptions in your life are positive and sustainable.
First, plan them. If you promise to only have exceptions that you planned at least week in advance, this avoids the risk of having a random cheat night where you break every rule because of temptation and greed. Then, don’t just plan one because you want to, have a valid reason. I think exceptions work best as celebrations. When you have a genuine achievement, do it. Celebrate. Because then, rather than treating it as just a ‘break’, you’re treating it as a mark in your journey. It makes the achievement official. It allows others to see it. And it tells you: “this is the start of the next chapter”.
Second, reduce the blast radius. What I mean by this is to only break the rules you actually need to. For example, let’s say you want to drink alcohol for a celebration. The alcohol itself isn’t even that damaging for a one-off, but the issue is that it typically leads to junk food, and a late night, and all sorts of other broken rules that make it a significant setback. If you make preparations like a pre-made clean dinner and a defined end time for the celebration, you might not feel any setbacks at all from the exception, or you might even still make progress during it!
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What exceptions are, they should be celebrations, don’t make them regular, have a consistent lifestyle instead that you always enjoy