Everyone's trying their best with what they've got. Sometimes you don't have the energy, information, innate ability, etc to make the absolute best choice. Actually many times. So you make the choice that is best according to your current circumstances and abilities.

That's not to say people can't improve. When they do, they've upped their level of "best they can do".

When I remind myself of this, that people are doing the best they can at that moment, it helps me forgive myself and forgive others for disappointing me.