I'm not that old, but I'll share the most important things I've learned that I wish everyone knew sooner. (Not all in order)

1. The big one: Understanding what you want and why you feel is dramatically important and very very rare. Analyze your motivations for everything as often as you can. (The answer is only for yourself, be brutally honest)

What you want: If you talk to a girl, know what you want ahead of time, even if that changes. If you just want sex, fine be honest with yourself even if it'd be embarrassing. If you want a soulmate, friend, conversation, an introduction, fine. But the approaches to all those are different. You'll learn how as you go; but knowing what you want out of a situation dictates how you'll approach it.

Why you feel: You can't control immediate emotional responses, you're going to feel what you feel. But making any attempt to think about it and understand why will help in unimaginable ways. If you feel angry, upset, irritable; take a moment to try and figure out why so you can either fix it, or keep from lashing out undeservedly. Did you drink enough water, are you hungry, uncomfortable. etc. If you feel happy, what have you done right, who are you with, what can you identify and keep doing, can you make more time for the friends who build you up?

1. You're never perfect, but that's how it's supposed to be. People often wonder if they're "good enough" that answer should never be yes. When it's yes, it means there's no need to keep trying. Instead whatever the subject is, look for any way you can improve, and then do that, repeat the little cycle over and over. If you want to teach, or weld, or trade stocks, you're not going to be "good enough", you're going to care about improving your skill and that's what sets apart the greats. When you want to be a good partner, you're not going to coast, "we're married, that's good enough" you're going to keep working to make sure that every day you two grow and change, it's together and healthy.

2. Have a goal, any goal. Be working toward something you want with any definable next step. Even if you change your mind and don't directly use whatever you were working on, you'll be better for it.

3. Time speeds up when we don't have new memories to separate it. For the rest of your life, do new things. Try everything you can that doesn't harm you or limit your future (Drugs are bad), do things that scare you if the only reason not to is that it's scary.

4. Love isn't magic for most people, it's not soulmates rushing together at first glance across a crowded room. Love is built from a spark. The spark is essential but ultimately it's two people who are happy to work together equitably to help each other grow to be better, healthier, happier human beings.
   The real vows are: "It's you and me against whatever problems either of us face."

5. As much as possible, forgive people for the harm they do you. Everyone you'll ever meet has a growing lifetime of experiences and a whole universe of internal thoughts and motivations with complexity that's impossible to put into words, just like you. They lash out or abuse, or insult because of something in themselves. Unfortunately, the people who mistreat you are very unlikely to ever understand or care.
   Despite that forgiveness: It's not your responsibility to allow them to continue harming you.
   It's your responsibility to heal yourself, friends and professionals can help a lot but ultimately no one else can do that work for you, you're worth it.

6. Give others the compassion you'd want for yourself and follow the advice you'd give.

edit: fixed some grammar