Ran across a great NY Times article by Gabriele Oettingen, a university professor of psychology. The title of her piece is "The Problem With Positive Thinking". Here are a few clips from the article.
"In each of these studies, the results have been clear: Fantasizing about happy outcomes — about smoothly attaining your wishes — didn't help. Indeed, it hindered people from realizing their dreams. Why doesn’t positive thinking work the way you might assume? As my colleagues and I have discovered, dreaming about the future calms you down, measurably reducing systolic blood pressure, but it also can drain you of the energy you need to take action in pursuit of your goals. ... Positive thinking fools our minds into perceiving that we've already attained our goal, slackening our readiness to pursue it."I suggest you read the rest of the article here. It is a good read and speaks to ways to balance positive thinking with a dose of realism. If this peaks your interest you might be interested in optimism defined as Proactive Realism.