Brett Steenbarger lists the top ten reason that traders lose their discipline:
Losing discipline is not a trading problem; it is the common result of a number of trading-related problems. Here are the most common sources of loss of discipline, culled from my work with traders:
10- Environmental distractions and boredom cause a lack of focus;
9- Fatigue and mental overload create a loss of concentration;
8- Overconfidence follows a string of successes;
7- Unwillingness to accept losses, leading to alterations of trade plans after the trade has gone into the red;
6- Loss of confidence in one’s trading plan/strategy because it has not been adequately tested and battle-tested;
5- Personality traits that lead to impulsivity and low frustration tolerance in stressful situations;
4- Situational performance pressures, such as trading slumps and increased personal expenses, that change how traders trade (putting P/L ahead of making good trades);
3- Trading positions that are excessive for the account size, created exaggerated P/L swings and emotional reactions;
2- Not having a clearly defined trading plan/strategy in the first place;
1- Trading a time frame, style, or market that does not match your talents, skills, risk tolerance, and personality.
Emotions are a performance and discipline killer. The difference in stress levels between managing money based on systematic models and using manager discretion is immense! You can spend a lifetime trying to make yourself immune to emotions (not going to happen while you are still breathing) or you can rely on systematic models. Systematic models have the benefit of never getting tired, upset, nervous, or bored. Furthermore, research in a variety of fields confirms the performance advantage of systematic models.
Posted by Andy Hyer 
