How do you stop overthinking everything?

The short answer: overthinking is not a thinking problem. It is a deciding problem. The mind goes round because something has been asked of it that it cannot deliver: certainty before a decision. It stops when you make the decision without the certainty, or when you admit what you are afraid of and look at that instead.

Everybody who overthinks has tried to stop by thinking about it. It is the one approach guaranteed not to work.

What the loop is doing

A loop of thought is a search for an exit that does not exist. You are looking for the version of the decision in which nobody loses and nothing can go wrong, and you will keep looking as long as you believe that version is in there somewhere.

It is not. Every real decision disappoints a possible future. The loop is what refusing to accept that feels like from the inside.

Why it feels like diligence

Because it looks exactly like care. Going over something again seems responsible, and sometimes it is. The test is whether a new round of thinking produces new information. If it does, think on. If the fifth round is the first round with different adjectives, you are not thinking. You are postponing.

The question under the question

Overthinking is nearly always about a feeling, not a fact. Under "should I take this job" is "am I the kind of person who can fail at something in public". Under "should I say this to her" is "can I survive being misunderstood by somebody I love".

The mind cannot answer those by reasoning, so it keeps generating the surface question in the hope that solving it will make the deeper one go away. It will not. Name the deeper one and the surface one usually becomes obvious, sometimes embarrassingly so.

Two things that work

Decide with a deadline. Not a vague one. A day and a time, said to another person. The loop runs on open time; close the time and most of it stops.

Or write the loop down, in full, every branch of it. On paper it is rarely more than a page, and seeing it at that size does something that thinking it at full size never can: it makes it look like what it is, which is three or four worries wearing a very long coat.

What it is not

Overthinking that has become constant, that keeps you from sleep, that is no longer attached to any particular decision, is worth mentioning to somebody who can help. Minds can get stuck in loops for reasons that have nothing to do with the question in them.

Where this is in the book

Midnight Reflection is the second scene of chapter four. A man stands alone in a shipyard after the hammers have stopped, with a vow he made to himself and a deadline closing on it, and asks the question he has been avoiding all day. The scene is about what can only be asked once nobody is around to hear the answer.