I do believe that the biggest psychological enemy in trading is frustration. Frustration comes hand in hand with perfectionism as it is triggered by the conflicts in our expectations and the achieved results. Frustration is that can quickly derail us and push into either an under-controlled or and over-controlled state of mind. When frustration shifts into anger that could result in really stupid decisions we would never ever make in a controlled and balanced state of mind..
Handling frustration is one of the most valuable tool a trader might have. There are a number of very useful posts on TraderFeed by Brett Steenbarger on this topic.