Being in hurry.
To find the right person, the right career, the right job. When you are young, you want it all to happen all at once, before you even have a chance to figure yourself out. Take your time.
Thinking that love is a feeling.
Love is a choice, and a commitment. It's something you do, not something you feel. Trust those who act like they love you, not trust those who say that they do.
Drugs and alcohol can be addictive, which means that they will make you do things you normally wouldn't do. Pay attention and be cautious with them.
Living to work, instead of working to live. Never pass up a good thing.
Cynicism. If you think you can't make a difference, you probably won't. There is no margin in being a cynic.
Thinking that your parents aren't as human as you are. And taking them for granted: they won't be around forever.