Investing and the stock market have transformed over the past 40 years. But, even with the best technology and free trading, good investing isn't easy. Stock market investors today make one big investment mistake that costs them money. And modern tools only make this mistake easier to make. Investors used to own their investments on average for 5 years. Today, that time frame has shrunk to 10 months. In this video, I break down what caused this, why its hurting your investment returns, and how to ensure you don't make this investing mistake.