Overthinking is a condition almost all of us suffer from in various doses. While a one-off instance of overthinking is not harmful, constantly getting your mind to overthink, can be detrimental in the long run, as it completely saps you of energy, and turns you into a negative person. However, the trick is to train your brain to not overthink, and in the process, use it to the best of your capacity. Here's how... (source: keyforsuccessmind)
To heal your brain, you have to connect better with your body. To manage this, take 3 intentional breaths, and feel your feet grounded. Then, try a cold shower, or 30 seconds of stretching to snap out of your mind's loop. This will help your brain divert to your body, and in the interim, the overthinking will be put at bay. Doing this repeatedly will recycle your brain to work better.
Overthinking occurs because of situations you can't really control, that is an exam result, some medical diagnosis etc. However, regardless of that, there are some things you can control, even if they are not related to the issue. For this, write down something you can control on a piece of paper (such as eating healthy, exercising etc), and then follow it up. Let the rest go, even for the next one hour.
Before jumping to conclusions of any sort, ask yourself these basic three questions. Will this matter in 5 minutes? Will this matter in 5 days? Will this matter in 5 years? Asking yourself these questions reframes the situation, and helps you differentiate the important from the unimportant, so that you can declutter your mind, and lead it to happy, healthy thoughts, even if temporarily.
Can't still help overthinking? Go ahead, but give yourself (or rather your brain) a deadline. Set a timer for 10-15 minutes, or any slot that you feel comfortable with. In this slot, worry as much as you want about anything under the sun, even if it is silly and small. Once done, switch off your brain, and keep an activity ready to be carried out. It could be anything from gardening to listening to music. Remember, you don't need to know all the answers.
This apart, you should practice mindfulness to keep anxiety at bay. The fact of the matter is that there will always be something to worry about, it is up to you what emotions you would want to feed your mind with. For this it is important to inculcate positivity, take care of your mind and body, and in general, stay clear of negative people, and emotions.