Overthinking has been a big problem in most of the peoples life. Gives you a negative attitude, affects your relations with friends, family etc. How can you possibly stop yourself from overthinking when probably you have no control of what to think?
multiply take action
This is the most important and perhaps the most difficult. However, if you want to change your life, you need to take action now. If you just think, this will not work. Talking about this might be easy, but it's a difficult practice. However, you have to motivate yourself.