Sorry, not mocking you.
Of course, it's a little levity, but, it's my way of expressing what you are looking for in a response to this thread. In short, I'm not sure what you are asking.
Are you asking why is it that we can't let go of disappointments, mistakes, humiliations, etc? Well, when you think about it from an evolutionary standpoint, our species -- and other species -- remember the painful more readily because it's a warning sign. The first time you touch a hot stove as a child and get burnt, the act becomes an indelible/painful memory. The memory stays with you as a protective mechanism so you don't touch hot things thoughtlessly again.
Now, psychologically, we also remember more readily the painful over the pleasurable. The mechanism is the same -- to protect us from future pain. But, it doesn't work as well in the psychological realm because it can inhibit our growth by making us Chicken Littles. We never take risks again. That's not good.
So, "don't sweat the small stuff" takes on significance here. We must learn to allow mistakes to be our friends. Mistakes, in fact, are HOW we learn. When you attempt your first scale, you make mistakes. Teacher steps in, advises; you correct mistakes, you learn and grow.
Seeing mistakes in this way gives the glib saying "don't sweat the small stuff" some meaning, I think.
Is this what you are asking?