Some people may tell you about how positive thinking improved their lives. Just because those people always have a smile on their face doesn't mean they don't have bad days. The difference between you and them is how you handled that bad situation.
Happy people tend to look at the silver lining of a situation, and usually that helps to make a bad situation seem a whole lot better. Even the worst situations have a positive angle, but you have to be willing to open your eyes and look for it. It's all too easy to get sucked into negativity and tell yourself that you're not good enough for that job, you most likely won't achieve that dream and so on.