Psychology counts for 60% of your trading success (Money management and your trading system count for the other 40%).
Imagine for a moment that you have a trading system that gives you 99% accuracy. Without the correct psychology, you will find yourself manually over-riding the system, not doing what you were supposed to do and failing to act when you were supposed to. That is what we term as impulse trading. Trading on emotion is one sure way to financial disaster.