I think people confuse positive thinking with mindfulness. Mindfulness can lead you to have more positive thoughts, but people with our disorder seem to have a faulty connection, or a switch that flips.
I do know that if I effectively practice mindfulness excercises (note the effectively), I can affect a change in my mood. And I've watched The Secret, and though I can't always control my thoughts I believe in the principle.
There's a Native American story I really like about a war between the two wolves that live within us all. One is all positive emotions; joy, love, fun, compassion, gratitude, peace, friendship, etc. The other is negative emotions; hate, jealousy, fear, envy, anxiety, depression, anger, etc. An old elder is explaining this to his grandson, and the child asks him which wolf wins? The elder replies, the one you feed.
I think I have gotten more effective at shortening the duration of my episodes because I have found a way to become more and more aware and mindfull of my thoughts. And by telling myself ' hey you're thinking (insert bad thing here) an awful lot... notice that?' it seems to stop the thought in it's track.
So I can't rant about positive thinking with ya'll

... even though it's supremely annoying when I'm stuck in it. I want to fight the good fight, because feeling good and thinking positive DOES feel better.