I heard somebody hear on Cardschat comment that confidence is a great personality attribute to successful play. I think it certainly plays a role in success and growth over time because without it, decision making or evaluating decisions becomes erratic.
One article I read was based on the complication of understanding how to evaluate performance.
It cited this can be very difficult without data sets. Yet, even with data sets, interpretation often goes without opponent tendencies and other various table elements. Further complicated by variance, coolers, bad beats and hindsight biases.
Having a strategy and defined process for decision making is critical. Perhaps even writing it out. By having these two elements, a person can investigate a particular area of play, better interpret data, recall mistakes and lessons better, integrate concepts and ask intelligent questions which they can seek answers for.