When you do not have a written trading plan, even though you have developed a plan in your head, it is too easy to drift away and go back to old habits. Having the written plan will guide you to making the right decisions. Consider the difference between knowing what has to be done and what you want to do. In trading, what has to be done is always the right choice, yet if you do not have a trading plan, you can easily decide with what you want to do, instead.