Affirmations- Helpful or Harmful?

Affirmations are positive thoughts.  They are carefully worded sentences that contradict a negative thought.  Affirmations, like hammers, are a tool and as such have no inherent utility except in how we use them.  You wouldn’t pick up a hammer and knock yourself in the head.  Likewise, it is not a good idea to force ‘positive’ thoughts upon yourself in a manic, urgent way.  So how do we work with them to improve our lives?