If postive thinking alone in the face of adversity pays dividends, it can also be argued that pessimism will help save your life by making you run away or hide from the adversity.
We all know about the overconfident individuals that perform below expectations because in their state of overconfidence, they ignored the minute details and overlooked the part that was supposed to make them excel.
To be successful, we need a healthy dose of all these emotions. We need the ability to identify what emotions is prevailing and be able to use it to suit our needs and motivate us to achieve that we want to achieve. Don't let optimism blind you, pessimism stop you from taking actions and realism to lead you into procrastination. Your emotions are like tools, use them to achieve the success your heart desires.