I believe that everyone is doing their best -- including my children. I don't know whether it's strictly true. I can't point to any scientific study, but it's my honest belief, and it serves me so much to think that.
It's important to realize that a person's "best" is not constant. It varies based on everything from how much sleep they got to their current emotional state. A person's best is not the same as their maximum potential under ideal conditions. It's the best that they can do in their current reality. And sometimes their best is, frankly, not very good.
For a physical example, let's say that under normal circumstances I can do 20 excellent-form pushups in a row before my arms fail. If I got a fantastic night sleep before and drank some zipfizz, I might make 23. But if I got a terrible night's sleep and did the P90X arms workout right before, I might make only 4 before I collapse. This is a physical example, but it holds true to any other measure of someone's "best" from grades to manners.
I know as a parent it's so tempting to say, "I know they can do better!" Yes, perhaps they could in theory do better, if all conditions were ideal. Or perhaps they have the potential to do better if they learn additional executive function skills or social skills or study skills. But if they could have done better in that exact moment, they would have.
I just finished reading "Island of the Sea Women" by Lisa See. In the book the author says, "To understand everything is to forgive." In other words, if we really understood every little factor that went into every single thing someone did, we couldn't help but forgive them because we would realize they were doing the best they could with what they had.
If your children are doing their best all the time and still not meeting essential standards, does this mean we just give up? Certainly not. This is an invitation to dig in and find out what needs adjustment -- sleep, diet, friends, executive function, coping skills, medication, classes, teachers, extra-curriculars, and life skills are all great places to check in.
If you can identify why your child is struggling, you can begin addressing the lagging skill or the emotional or physical circumstance preventing them from doing better. If you're frustrated with a child, try asking yourself, "What is the most loving thing I can do right now?" This question will refocus your brain from disappointment and blame to understanding and curiosity -- giving you more ideal conditions to help you parent at your best.