There's a famous Russian tale about a young boy who is one day challenged by his brother. "Go to the corner and stay there until you can stop thinking of a white bear." The boy walks over, innocently, and wonders what to do next. He's not supposed to think of a white bear. Should be simple. After all, he wasn't thinking of one to begin with. He sits down and tries to set about not thinking of a white bear, when suddenly, he can't stop thinking about them. It seems the very command not to think of a white bear has condemned him to do so. His brother comes back ten minutes later and asks if he's thinking about a white bear, and sure enough, he still is.
The older brother in the story is sometimes said to be Dostoyevsky. Others insist that it's Tolstoy whose older brother sent him to the corner. If we take it allegorically, any of us could be the younger brother.
Most of us have heard the advice to "just stop thinking about it" when we confide that we are bothered by something. But has this advice ever helped? Surely, when we're better, we won't be thinking about it, but is it as easy as just deciding not to? Of course not.
Consider the advice to "not think about a white bear." How can one both hold this advice in one's head and not think of a white bear at the same time? Listening to this advice could not possibly work, for a very simple reason. The statement itself refers to and contains the thought you wish not to think about. If you are thinking about the advice, it guarantees you are thinking about a white bear.
One thing seems clear: when confronted with an unwanted thought, "stop stopping."
