If just stopping worked, you would have done it years ago. The advice fails for a reason that is worth understanding, because the reason points directly at what does work.
The white bear problem
Try, right now, to not think about a white bear. Notice what shows up. The mind cannot process a negation directly, so when you tell it do not think about that, you hand it the very thing as an instruction and drag it to the center of attention. Overthinkers who try to suppress a worry usually end up overthinking about their overthinking, a spiral about the spiral. Willpower is the wrong tool because the harder you push a thought away, the harder it pushes back.
You cannot will a loop off, but you can change your relationship to it
The thing that actually helps is not stopping the thought but changing what you do with it. This is the core of cognitive behavioral therapy (CBT). As the NHS explains, CBT helps you change how you think and act, and it is a first-line approach for anxiety and related problems. The point is not to empty your head. It is to catch a thought, question whether it is true or helpful, and answer it with something more balanced, which is a learnable skill rather than an act of force.
Underneath that is a simple, freeing idea, backed by the basic model of CBT summarized on the NCBI Bookshelf: your thoughts, feelings, and behavior are connected, and thoughts are not automatically true. You can notice a thought without believing it and without obeying it. That small gap between having a thought and buying it is where all the freedom lives, and it is built with practice, not with clenched teeth.
Why willpower feels necessary and backfires
Overthinkers often believe that if they just tried harder to stop, they would. So each failure to stop feels like a personal weakness, which produces more self-criticism, which produces more to ruminate on. It is a trap with a hidden exit: you were never supposed to win by force. The people who quiet their minds are not the ones with the strongest grip. They are the ones who learned to hold their thoughts more lightly.
The lived experience of this is familiar. In one widely read essay, a writer describes lying awake at 2am with a restless, perfectionist mind that will not switch off no matter how much he wants it to. Wanting it to stop, however badly, is simply not the mechanism. The mechanism is different, and gentler.
What to do instead
Rather than suppressing a thought, you acknowledge it without engaging: oh, there is that worry again, you can be there, and then you gently move your attention elsewhere without arguing and without obeying. You are not fighting it and not following it. That starves the spiral of the engagement it runs on. We build this out in catch, challenge, reframe, and in the daily practice of noticing in how mindfulness helps.
The scheduled alternative to suppression
One counterintuitive move works far better than white-knuckling: give your worries an appointment instead of a fight. Pick a fixed fifteen or twenty minute window each day, and when a worry shows up at any other time, do not wrestle it and do not obey it. Note it down and tell it, not now, I will give you your full attention at six. This does two things. It reassures the anxious mind that the worry will be dealt with, so it stops nagging, and it moves the worrying out of the whole rest of your day into a contained slot. The quiet punchline that most people discover is that by the time the window arrives, a large share of the worries have deflated to nothing, and you sit there mildly amazed you were going to let them eat your afternoon.
That is the pattern with almost every tool that actually helps. None of them is about forcing the thought out. They are about changing what you do with it: postponing it, questioning it, letting it pass, or acting on the real problem underneath. Force keeps you locked in a struggle with your own mind. These moves step out of the struggle.
So the honest answer to how to stop overthinking is that you do not stop it by trying to stop it. You change your relationship to your own thoughts, one caught thought at a time, until the loops catch you less. That is the whole project of The Overthinker's Manual, and it starts with dropping the one strategy that was never going to work.