The art of positive thinking is a beautiful thing. However, I think the biggest misconception about positive thinking is that you have to do it ALL the time, and any negative thoughts are bad thoughts.
This is simply not true.
Bad things happen, accidents happen, relationships end, people get rejected, cancer happens, people get laid off, people pass away. There are plenty of "bad" things that happen in this world. This is exactly where I find positive thinking to be most valuable. Allow yourself to feel negative emotions, deal with them and move on knowing that the good always comes back around. It's only when you let the negative overtake your life does it become a problem. We live in a society where negative emotions are masked with anti-depressants, alcohol, over eating, shopping... the list goes on and on. Not one of these coping mechanisms is worse or better than the other. They all just are. Incorporating positive thinking makes the pain less painful because you realize it's only temporary. It's okay to feel that way today, you won't feel like that forever. Go ahead... cry it out.
Don't think you have to see the silver lining in every cloud. Just because you can't see it doesn't mean it's not there and it also doesn't mean that you still don't feel the rain.