The battle for trading success primarily takes place internally for the trader. The external results of long term returns are generally caused through behaviors that arise from within the trader themselves.
Most successful trading systems arise due to the discipline of the trader to do the work to create the robust system. Robust trading systems arise from an internal work ethic not luck.
Profitable traders have learned to fight and win over their ego and stop trading so big and letting losses run hoping they will come back. Controlling ones ego is an internal job that can make a trader profitable.
Traders that make large returns in markets with favorable conditions are able to keep those returns because of the internal discipline to stay with their trading plan when the market dynamics change. They do not give back their profits.
Traders that are profitable long term are those that have learned to not trade based on their own fear and greed but trade off of others over reactions. They are not controlled by their own internal emotions but have changed and trade based on actual external price action based on their signals.
Profitable trading is an inside job. It takes strong internal anchors to do what is right to be able to sail through the external storms of price action and reach the shore of profitability.