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?
Find space and time to clear your mind. Some people call that meditating. Silence is underrated. Focusing on your breathing will help you to stop thinking (thinking words anyway), having the patience to sit and be in silence is another thing altogether, takes practice.