The hardest poker lesson, and one that takes years to truly internalize, is the ability to completely separate the outcome from the quality of the decision. It didn't take me years, but I quickly learned that you shouldn't get upset when you play badly, regardless of the outcome, and that you should be okay when you lose money, as long as you played correctly.
The hardest poker lesson for many players is learning that good decisions are more important than short-term results. Success comes from focusing on making the best play, regardless of whether you win or lose a particular hand.
