There are many things I have learnt in my 20 years on this earth.
1. Everyone is going to screw you over. 2. Things happen for a reason, and if they don't, it just isn't meant to be and something better is waiting. 3. Karma is a bitch. 4. If you are waiting for something to happen, you'll be waiting a long time. 5. It's the choices that make us who we are. 6. Never let your heart rule your head if you can help it, unless they agree. 7. Never plan life out. Thing's will happen when they're ready.
truely self confident people are not what you describe, bullies and arogant people are proble the least self confedent people. That said the most important lesson people need is to learn the difference between what they need and what they want.
Well, since enough people have lectured you on the confidence thing already, I'll just add my own answers and hope others do the same since I think it will generate some interesting responses
Not quite a lesson, but I think everyone should be made to do a "crap job" at least once, preferably a few times throughout their life. Even if it's just one or two weeks every 5-10 years, just to help people retain at least some respect for typically undesirable jobs that are much more important than people think. Maybe people would care more about not-littering if they had to clean up trash other people carelessly dumped on the ground for a week. Maybe people eating out at restaurants will be a little more patient and understanding about why their order is taking so long when they're slaving in a hot kitchen trying to feed 200 people every hour. In fact I would say this is especially necessary with people in elite jobs who lost touch with the common working man. So easy for them to have a smug grin complementing their clean suit in the company CEO photo when they get paid four times as much as their employees who spend all day cleaning floors, carting heavy crap around or copping abuse from whiny customers just so they can in 15 years time fully pay off their house.