I bought my first stock 21 years ago. $S&P 500(.SPX)$ Here are 21 harsh investing truths I learned the hard way:1: The worst mistake is to sell a mega-winner earlyImage2: Humans are pre-programmed to be bad at investing.3: Your personal finances are 10x more important than your investments.4: Handle volatility is 100x easier in theory than in reality.5: Confidence in your strategy will rise and fall in lock-step with asset prices.6: The best stocks put their owners through gut-wrenching volatility. The worst stocks do, too.7: You're going to be wrong—a lot. Be humble.8: It's never as good as it seems or as bad as it seems. 9: Successful investing requires an uncomfortable mix of pessimism and optimism, which is damn hard for our brains to hand