Wisdom you weren’t given “back then” but should have been

Is there something that would have helped you in your life, if you had known about that?  Something that would have saved you time and effort and a lot of heartache?  Here are the most voted of all answers by the audience of this question and the responses it got.

• Marry your best friend: If the person you’re the most comfortable with isn’t your spouse, make them! (Assuming this person is not your family member)
• Don’t try to be a “grown up”: Always have fun. Mud-wrestle, sing, pillow-fight, at any age.
• Don’t stop learning: If you start coasting through life, you’re gonna lose. Always stretch your intellect.
• Don’t always try to be original: Just tell the story or paint the canvas or whatever.
• Focusing on “fairness” will lead to stagnation.
• If you’re not failing, you’re doing it wrong. (It’s OK to make mistakes.)
• Don’t try to reason with mindless, irrational people.
• Don’t stress yourself out with news and “staying informed” too much.
• Do something that’s not for money.
• The key to happiness is BUILDING stuff, not GETTING stuff.
• True love and a good marriage involve a lot of fighting, not serenity.
• Time passes by a lot faster than you’d think. This effect accelerates with age.
• Wealth is relatively unimportant.
• Some things can’t be learned; they can only be experienced.
• Figure out who you are, then ACCEPT that person, and then BE that person.
• Don’t wait for permission. Give yourself the okay.
• Don’t lie to yourself.
• Forgive as much as possible. Grudges achieve little.
• Be humble (especially to the “little” people).
• You and you alone control how happy you allow yourself to be.
• Find a mentor and BE a mentor.
• Find what you like and let it kill you.
• If you have to go to the bathroom, stop what you are doing and go. Don’t try to “hold it.”
• You don’t have to eat everything that’s on your plate.
• You don’t have to pick up a phone that’s ringing.
• Always take action on things. People regret inaction more than action.
• The past is something you learn from.  It is not something you live in.
• Wealth is measured by your happiness and not by your financial statement.
• Your mind decides what is hopeless.  Your circumstances do not.

And here is an answer which has a series of illustrations which make some of the most interesting points:
Read Quote of Aurélie Ré’s answer to Life Lessons: What are important things and advice to know that people generally aren’t told about? on Quora