You're exploiting your own scope neglect bias: You know you'll regret what happened at that event, but you also kind of know that you'll regret it a similar amount, whether you eat that additional cookie or you whether you don't.

Moreover, eating that additional cookie lessens the weight assigned to the first transgression (e.g. the pizza), because now instead of a singular event it's merely one in a series of events, and thinking about the whole series, there's less individual regret based on that first pizza incident.

It comes down to trying to game your own reward/punishment system, acting as if that surrogate parameter is the actual goal (instead of the weight loss).

Puny hu-mans!