Thought stopping is the habit of watching one's own thoughts and deciding when they are helpful or not. If they are not, you can literally say to yourself "I chose not to have this thought" and then actively decide to think other thoughts. It is a deliberate not going down an unhappy, obsessive path inside your head.
A lot of people feel they cannot do this simple thing. But in most cases, they can.. Painful and intrusive thoughts are often the result of habit, pure and simple. There is no reason you have to pay attention to a particular thought just because it wants to play in the field of your mind.
Habits are ruts. You know you are in a rut with your thoughts when your thinking does not resolve a hurtful problem or situation.. You can also tell that you are caught in runaway thoughts when your your friends and family are not as sympathetic or patient with your problem as they once were.
So try redirecting your unhappy thoughts, over and over again until you find one day you are thinking of them a lot less. If you try this without success, you may want to try going to therapy. A supportive trained professional can give you the tools to support you in moving more quickly through your problems