Confusing trading with investing
The mistake
Trading means buying and selling fast to profit from price moves. Investing means buying good assets and holding them for years. Most people who try fast trading lose money — study after study shows it.
The fix
Decide upfront which one you are doing, and size your money accordingly. Long-term investing money goes into diversified funds or bonds and stays put; anything you genuinely want to trade should be a small amount you have explicitly decided you can lose.
A concrete example
An index fund position bought and held for a decade captures the market's broad long-run growth with minimal effort; the same money moved in and out weekly chasing short-term moves adds trading costs, taxes on each gain, and — for the large majority of retail traders — a worse result than simply holding.