If you're an overthinker, here are a few things that can help:
1) Meditation -- Take a few minutes every day to practice clearing the mind. Over time, it helps, but in the meantime ...
2) Triggers -- Figure out what triggers you to overthink: Is it a need to control things? Is it co-dependency & feeling like you're supposed to take care of everyone? Is it a critical inner voice that starts telling you how you screwed up or how you're not good enough?
3) Pay attention! -- Try to catch yourself when you start ruminating, before you start going into that downward spiral. Notice what triggers those thoughts (that's where meditation helps).
4) Stop yourself from going there! -- Find a distraction, call a supportive friend, take a bath, watch a movie - anything that stops the auto-pilot & redirects you to go down a better path.