Dear Andrew,
Negative thinking can be one of the most destructive things you can do. It requires great effort to change this negative thought pattern. However, when we learn how to change these negative thought patterns to positive ones you will experience relief and a feeling of being more resourceful.
Positive thinking is about choice. You can always determine whether you want to be happy or not. As I see it your first step is to make that conscious decision and every time you catch yourself in thinking negative you stop it immediately. Try and read my post on this: http://andershasselstrom.com/today-i-choose-happiness/
Hope it helps Andrew,
Best,
Anders Hasselstrøm