Only in hindsight you can see what really happened with your trade, so you can know what you do (follow your system), but you never know the outcome in advance, so in fact you don't know what you do. That's the reason why some people have fear. Depends also from personality. I had a long time fear to trade, even when there was a very high probability that I would win big. So mathematically nothing could go wrong. But it went wrong emotionally.
Emotions can block even very successful traders.
Example 1: I am afraid standing on a roof of a 50 store building. Mathematically the possibility that something can go wrong is almost zero. But I am still afraid, mathematics cannot take away my fear. If I receive mental coaching maybe I will be able to control this fear.
Example 2: A young elephant was tied up with a thin rope to prevent him from running away. The young elephant tried many times but did not succeed. Many years later the elephant was still tied up with this same rope. He did not try to escape anymore because he was convinced that after trying so many times already in past he would never succeed.