What's the biggest problem you or someone you know faces as a trader that still doesn't have a good solution?

We hear a lot about winning strategies, but not enough about what actually makes trading difficult.

What's the one thing that repeatedly costs traders the most?

Whether it's losing money, wasting hours every day, missing good opportunities, bad risk management, emotional decisions, or something else entirely...

What's the biggest problem you've faced that you still think doesn't have a good solution?