Emotions!

"Whether you believe it or not, it is a fact. Many traders psychologically sabotage their trading and they do not even know it. They are not prepared for the flood of emotions that sweep over them when they are trading and, therefore, they are unable to deal with them. 

When the bank account balance is on the line, you will experience greed, fear, and denial. You will react in ways that you did not anticipate and, at least part of the time, you will take irrational and self-destructive action. In order to succeed as a trader you must prepare for the worst, just as a jet pilot must prepare for an engine failure. You must recognize the emotions that you will experience when you are in both winning trades and losing trades and you must be prepared to deal effectively with them. Design specific strategies for the market’s movements and be ready to execute those strategies when your emotions get fired up. If you do not recognize the power of emotions in trading and if you do not prepare yourself to effectively deal with them, you will fail. 

Do not destroy your next trade by worrying about and reliving your last one. If you plan to be a long-term player, there will be hundreds and maybe thousands of other trades."

-- Winning the Day Trading Game, Thomas L Busby,
Chapter 5 "There's No Crying in Trading"

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