There are several important elements of psychological part of trading process.
First of all, trader should be able to analyse mistakes and make right conclusions. Each time losing money he has to figure out the exact reason of loss - his particular action that caused such results. Then, he has to define a way to avoid situations like that in future. Without such analysis it would be almost impossible to become successful trader despite any knowledge or experience.
Second one, but not less important element is discipline. Trader should be disciplined enough to act strictly in accordance with the rules prescribed by his trading system and avoid any emotional trades. If the strategy is profitable, it would be possible to make profit using it, while any other actions may lead to losses. The other rules the trader shall follow may include maximum loss per trader and per day, limit on position size and list of conditions prohibiting trading in general like elevated volatility due to the news or technical issues or problems with network connection.