Things I wish I knew when I was 20, but if I did, I wouldn't have cared anyway. Pity.
Happiness is growth
You need to grow to be happy. Grow physically, mentally, spiritually, intellectually, socially, or in whatever other way that matters to you. But keep growing. Improve your environment. Improve your skills. Work on your talents. Help others grow. Read up on things you don´t understand, like politics, the middle east crisis, or how an automatic gearbox works. Learn, explore, discover, experience. Wikipedia is a great place to start. When you stop growing, you get bored.
Don't get bored
When you are bored, your mind wanders. To dark places. That leads to depression. Depression can kill you. Don't get bored. Grow.
Accept it and move on!
When things don't go as planned, accept the situation immediately. Then move on. The quicker you accept the situation, the better. Watch Prison Break to understand how powerful this can be. This is what got me through three engine rebuilds on my car, when most people would have sold it.
It's easiest to shape up in your 20's. From 30 onwards, it's an uphill struggle. Your metabolism slows down, leading you into a viscious cycle of weight gain and bad eating habits. Accept it and move on. Learn how to increase your metabolism. Learn from body builders, they are the experts. Except without the steriods and other enhancers. Do anerobic excercises. All it takes is 3 hours a week and some healthy eating habits. Being in good shape boosts all aspects of your life.
Learn about cholesterol
Your body produces it's own, so you can have high cholesterol even if you don't eat any. Or vice versa. Eat cholesterol-reducing foods (good fats). Exercise. Get checked up.
Build your wealth
You are getting poorer. Unless you have investments that beat inflation. Inflation is higher than you think. It's more than the bank will give you on a savings account. Debts are also making you poorer. You could be financially independent (i.e. Retire) by the age of 40 just by investing R1000 per month from the age of 20 into an investment that gives you 15% return per annum. That kind of return doubles your money every 5 years. The stock exchange can give you that kind of return. Invest in the stock exchange. Learn how. Start with unit trusts.