Why do I overthink everything?
You overthink because some part of you learned, at some point, that thinking hard enough could prevent a bad outcome, and your nervous system has never been told that job is done. It is not a character flaw or a lack of discipline. It is a strategy that worked once and never got retired.
The loop is doing a job
Overthinking rarely feels like choice. It feels like a door you can't close, replaying a text message, a tone of voice, a decision you already made three hours ago. Underneath that loop is usually a prediction problem. Your mind is trying to simulate every branch of a situation so nothing catches you off guard, because somewhere back there, being caught off guard cost you something real. Maybe it was a parent whose mood you had to read before you walked in the room. Maybe it was a job where one missed detail got you blamed for the whole project. The loop is not random. It is pattern matching against a version of the world where vigilance kept you safe.
Why it makes sense, not why it's broken
A brain that overthinks is a brain that has been rewarded, at some point, for catching the thing everyone else missed. If you grew up around unpredictability, thinking ahead was not neurotic, it was accurate. You needed to know which version of a person was walking in the door. If you have ever been blindsided by a betrayal, a diagnosis, a layoff you didn't see coming, your mind filed that away as evidence that not thinking enough is what gets people hurt. So it compensates by never stopping. This is adaptive logic running on outdated data. It made sense then. It is costing you now, because most of the rooms you are in today do not require that level of scanning.
What actually helps
Trying to stop the thoughts almost never works, because you are fighting the same mind that generated them, and it will just generate a thought about how you are failing to stop thinking. What tends to work better is giving the loop a job with an end point. Set a timer for ten minutes and let yourself think as hard as you want about the thing, on paper, then close the notebook when the timer goes off. This works because rumination without a container feels infinite, and infinite is what your nervous system reads as danger. A second thing that helps more than people expect is asking what decision the overthinking is actually protecting you from making. Often the spinning is a way to avoid committing to a choice you already know, because committing means you can be wrong and blamed for it. Naming that out loud, even just to yourself, tends to shrink the loop faster than any breathing exercise.
The body is part of it too
Overthinking is not purely mental. It usually shows up alongside a tight chest, shallow breathing, a jaw that won't unclench, because your body is bracing the way it would for a physical threat. If you only address the thoughts and ignore the physical bracing, the loop has somewhere to live and it will keep running. Even something as basic as pressing your feet into the floor and naming five things you can actually see in the room can interrupt the simulation your mind is running, because it forces attention back into a body that is, in this exact moment, not actually under threat.
When this runs your life, it usually traces to one underlying pattern. For this, it is most often the Mirror rhythm, the thing under the behavior.