The way to stop is to not associate to the negative thoughts. Everytime you ponder deeply about a thought or emotion, you make it stronger.

I have bad depression. I have dark fears. Anger issues. But the way I deal with these thoughts is to not indulge in them.

The reason meditation helps me is because it distracts my ruminating thoughts by focusing on my breath. The way to deal with rumination, is distraction.

The next time you have a negative thought or feeling, think of something else or squash it as soon as you can. It will grow stronger and seep into the unconscious creating tension in the body.

Try to find some type of distraction. Hang out with people more. Go outdoors. The more you are in isolation, the more likely the rumination will come again.