At a training I attended this story was included in our materials.

An Old Cherokee Indian was speaking to his grandson: “A fight is going on inside me,”he said to the boy. “It is a terrible fight and it is between two wolves.  one is evil–he is anger, envy, sorrow, regret, greed, arrogance, self-pity, guilt, resentment, inferiority, lies, false pride, superiority, and ego.  The other is goo — he is joy, peace, love, hope, serenity, humility, kindness, benevolence, empathy, generosity, truth, compassion, and faith.  This same fight is going on inside you, and inside every other person, too.”  The grandson thought about it for a long minute, and then asked his grandfather, “which wolf will win?”

The old Cherokee replied simply, “The one you feed.”

Source: Whittmer G. Willkers (as noted on the story I have but it appears to be on the Internet in many places but usually just states Cherokee words of wisdom.)

Take some time and reflect on the part of your soul that you are feeding.  It seems to reason that if your focus is consistently on all that is not OK, the parts of yourself you do not like, and your mistakes etc., that  you will be inviting more darkness and unhappiness into your life.  Conversely if you focus on the amazing parts of your being and embrace that which is good that you will invite a more positive energy into your life.