Why can’t I stop thinking?

The reason why you can’t stop thinking about it might be because your brain lacks a specific chemical that allows you to suppress unwanted thoughts. This is according to a study that explored the region at the front of the brain known as the prefrontal cortex — the area that plays a key role in controlling our actions and thoughts.

Are you female and between the ages of 10 and 30?  The sort of mental fugue you are describing seems to be a fairly common problem in that demographic, perhaps due to diet or fatigue.  Try exercising (like going for a 30-minute run-walk) once or twice a day until you notice that you're sleeping better.  And make sure you're eating right - no dieting or over-snacking nonsense.

If that doesn't work, discuss it with your doctor.

(BTW - everybody thinks constantly.  We ALL are thinking, every moment of our conscious lives.  So you don't actually want to STOP THINKING...you just want to stop returning to one particular worry so much that it becomes tedious.)