1 - Do what makes you happy - I spent (spend?) a lot of my time doing my best to make people happy. I wouldn't do away with that part entirely, but I would remind myself to consider what I want.
2 - Stop trying to be cool - This one is harder to get across, because it's tough to prove the absence of something. Growing up, I was sure my every choice, every word, every action, and every emotion was being meticulously reviewed by SOMETHING, to determine if I was cool. This mythical judicial committee does not exist.
3 - Hope is good. I've found that having hope is considered by society to be "silly" in some instances. I should have held on to hope, and there isn't a time when it isn't awesome.
4 - Other people are interested in what you're interested in. I didn't have any interests which completely ostracized me while growing up, but living in pre-internet society in Canada led me to believe that society was boring. Hockey is fine, and wrestling is passable if you're looking to justify watching a soap opera with different characters, but I wanted lazers, space ships, and robots ALL THE TIME. Pre-internet, outside of my group of friends, I didn't think I could share those things with other people, and worried about that. I didn't need to.