Lots of good advice here and I am not THAT old but here are a few things I wish I had realized sooner in life (in no particular order or theme):

• No one really knows what they are doing, more experienced people usually have just made the mistakes already.
• There is a difference between being correct and getting someone to agree you are correct, the former is the most important and the latter is not always possible or desired and when to argue and when to let someone be wrong is a skill that takes practice.
• The less time spent worrying about what other people think of you the happier you will be and the easier life will feel. In almost all situations you spend more time thinking about whatever it is than the other person does.
• Always strive to be a good and like-able person but always remember that it’s impossible to be liked by everyone, sometimes people just won’t like you and that is okay.
• One friend you can count on is more valuable than 100 acquaintances, guard and nurture your valued friendships like your life depends on it because it might some day.

The biggest thing though, there isn’t one correct way to live your life and anyone telling you differently is probably selling you something. Always try to learn something from every situation and you will be fine, mistakes are a part of life. Anyway, hope at least some of that is helpful!