If you ever find yourself working really hard to convince yourself of something… don’t.
It’s barely ever worth it to try to convince other people of stuff. When it comes to yourself, don’t use motivated reasoning. That’s when you (emotionally) want a certain outcome, so you try to figure out which chain of logical-sounding arguments will get you there, rather than taking the facts and your core values and going where they lead you.
So you’re trying to convince yourself that it’s okay to eat that extra helping of cake. You want the cake, so you come up with all sorts of nonsense like “Well, I did take the stairs at work today…” Stop. Stop convincing. Here are the facts: the extra helping of cake represents a net health loss. Here is a core value: you desire health. The math works out pretty easily for one side, here.
You know the right answer, or you wouldn’t be having the mental wrestling match with yourself to begin with.