Negative thoughts feed on your attention. One way to deal with such thoughts is to starve them out by not giving them any. As soon as your mind throws up something ugly, ignore it and get busy with something else.
You have the power to choose how you respond to your thoughts, when you feel a worry nagging for your attention get up and move and and find something else to focus on.
Anything that you find engaging and relaxing will help you put your attention in a better place. Take a walk, call a friend, read, do a crossword puzzle. Just find something else to do to divert your mind and help you gain distance and perspective.