One day I realized that having patience and focusing on gradual improvement rather than quick fixes was necessary for deeply ingrained problems (like forms of akrasia).
Some others:
Taking breaks is not negotiable.
Maintaining social contact with someone is your responsibility, not theirs.
If you try to have too much control in your life, you'll become brittle and unable to handle unexpected changes.
"Willpower" is basically worthless. The best productivity strategy is to make productivity habitual.
4.5. You spend the vast majority of your waking day completely unconscious, doing whatever you're habitually used to doing.
Adopting political labels is a negative sum game.
Sometimes our minds suddenly "click" and we see a topic in a new light. Or sometimes we think we understand an idea, think it's stupid and ignore attempts to explain it ("yeah, I already know that"), until we suddenly realize that our understanding was wrong.
This kind of insight is supposedly hard to transmit, but it might be worth a try!
So, what kind of important and valuable insights do you wish you had earlier? Could you try to explain briefly what led to the insight, in a way that might help others get it?