You are what you think. Whatever you are doing, whatever you feel, whatever you want—all are determined by the quality of your thought. If your 'thinking' is faulty, it will lead to many disappointments and unhappiness. If your thinking is overly pessimistic, it will deny you due recognition of the many things in which you should properly rejoice.
Try this out; identify some examples of your strongest feelings or emotions. Then identify the thinking...
