I can easily put a negative thought out of my head

A thought arrives without knocking and there is no mechanism for refusing it entry. What there is, is the option not to answer.

Most of what makes a bad one grow is the reply — the counter-argument, the evidence gathering, the twenty minutes spent proving it wrong, all of which is attention, and attention is food.

Leaving it unanswered isn't agreement. A thought sitting in the room unengaged is a very different object from one you have been debating since Tuesday. It goes flat. Then something else takes the space, usually something dull like remembering you need milk, and later you can't recall when it left.