Overthinking might seem impossible to stop specially if you've continuous stimulus coming from your external world (e.g family, relatives, work, etc). However it is possible and you get better as long as you keep doing.
Here's the list of what's been proven to work:
1 - Breathe: it basically consists on being more conscious about it. Pay attention to the rhythm. Better slower than quick breaths.
2 - Youtube's videos: search for relaxing videos instead of entertaining videos. Some videos might be too exciting which are the ones to avoid. Others are relaxing such as watching fish swimming, AMSR, etc. It's proven that heart rates and blood pressures drop.
3 - Regular physical activity: One day per week is not enough. You must do regular exercise to see relevant benefits. Aerobic exercise has been proven to release endorphins.
4 - Listen to music: it literally calms the activity in your brain however avoid songs that evoke vivid memories. Listen closely, not just as background. The more you notice, the less you'll dwell on your other thoughts.
5 - Get a diary: write down all your thoughts on paper before going to sleep. Next morning they all will be written there so you can empty your mind at night. You can continue next day with your "thinking plan".
Obviously there are others way to calm your mind. Just find which is the most suitable for you and try it every day. The first day might be hard to relax yourself, but tomorrow will always be easier.
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