There is one simple and easy way you can stop overthinking by asking yourself questions about your conditioning of overthinking.
This is what I follow through to stop overthinking :-
Getting involved in some work.
I talk to someone or myself about plans and to-do lists.
Asking questions like: is this really important or just sucking up my energy and time ?
Whenever I am in free time, I write whatever I feel or think everyday.
Now coming to the point, how to have a positive attitude ?
You must avoid interacting with negative minded people or friends.
Read an article which has a positive approach to life.
Put a smile on your face and learn to make up some jokes, So others also will also laugh when they are around you.
Do not watch Bollywood's shit.
Have a positive approach towards others and your words.
If you have any suggestions let me know...