Many people confuse ‘remembering’ with ‘thinking’.  Instead of running memories through your mind or constructing thoughts and movies in your head of ‘what terrible thing might happen if….’, get into the habit of thinking on purpose.  

When you catch yourself running through a bad memory or seeing the worst that could happen or constructing a fear in your head (which is usually based on a memory), STOP!  Take charge of your own mind and direct your thinking to what you are doing at that moment in time (focus), or if you want to think ahead, make sure you are thinking on purpose and making good plans in your head.  The more you do this, the more it will become your normal automatic behaviour and the more you will be able to enjoy life.