This is not related to the pseudo-scientific Law of Attraction. In fact, the idea that a person can simply wish, think, or feel something into existence is completely preposterous, unsubstantiated, and dangerous. Be careful to avoid pseudoscience wrapped up in slick marketing.
One proven method, however, to go from a negative, depressed pessimist to a positive, happy optimist is through Learned Optimism. This technique was developed by Martin Seligman, a pioneer in positive psychology and his research postulates that when a depressed, pessimist adopts this new style of thinking, they can significantly improve their condition. He found that optimists tend to be higher achievers and have better overall health. While pessimists are more likely to give up in the face of adversity or to suffer from depression. Seligman invites pessimists to learn to be optimists by thinking about their reactions to adversity in a new way. This change in thought is what he coined an Optimistic Explanatory Style; essentially a new way for a person to interpret and react to life events. This style is centered around 3 key conditions: Personalization, Permanence, and Pervasiveness. For example, an optimist's outlook on failure can be summarized as "What happened was an unlucky situation (not personal), and really just a setback (not permanent) for this one, of many, goals (not pervasive)". So you see, the optimist doesn't dwell on things and let them fester. They identify the issue, process it, and move forward in a positive direction.
As if that wasn't enough, new research is now correlating cynicism with an increased risk of dementia.
We have control over how we think and therefore how we feel. This is the essence of TREWTh: we conquer our selves in order to live our best lives. In order to do this, however, we need to get out from under the heavy burden of our distracted lives. We need to stop constantly seeking pleasure and avoiding pain. Then we are free to focus our attention within where we can let go of our ego and begin to embrace our true selves. This is why the practice of meditation is so powerful; it forces us into quiet solitude so that we can learn to master our thoughts.
Go, take control and start thinking right!