It's important to be harsh on yourself

This is a blanket statement that doesn't apply to everyone. For some, shame and the self-esteem lowering thoughts that come with relapse can set them even further back. Lapse of self control is also not the reason a majority of folks relapse, since a majority of the people here have some form of addiction. It's more about not having the proper tools to deal with the addiction than any internal weaknesses.

 

This isn't the same as breaking your diet for a normal person. It would be similar if, in your pizza example, the person was a food addict and they had a binge day. They fell off the wagon, yes, but for many addicts, the shame you describe as helping themselves will make them relapse more.

I'm not saying they should lie to themselves about the situation. They should be honest and critical in a constructive manner. Why did they fall short? Were there triggers? Was there something that could've been avoided? How? What's the plan for success next time?

Tools to succed are much more effective than shame. There's an NFL coach who likes to say, "so what. Now what?" We can't change what happened in the past. We can only look to do better in the future.