These are the most difficult things I've learnt/am still learning.
1.There are always going to be people who are smarter, better looking, more sociable, and just all around "better" than you. In fact, you'll learn there are LOTS of people who fit that description. To be happy, then, you have to learn to accept yourself and your shortcomings.
2.Success does indeed come from hard work. Hard work without talent doesn't ensure success, but talent without hard work ensures mediocrity
3.It's unlikely you'll become anything close to your wildest ambitions, simply because you'll learn everyone has big ambitions, and there's only so many slots in this world.
4.Idealism aside, money = opportunity and therefore matters. Relationships and power matter more.
5.Friendship isn't forever. Social and economic status does create divides and strains in even the oldest relationships.
6.Your parent(s) basically gave up their freedom to raise you, and deserve a medal. You know this because with only one job on your plate, you already have no time.
7.Adult life is about learning to live with ambiguity. Remember when you thought you'd have it all sorted out by 30? No. Whether it be your career, your relationships or your beliefs, you will always question whether you are making the right choices and will have to deal with regret.