How to stop over-thinking? i always think about the bad side of things, and i want to stop.

Psychologist: You may want to enter into therapy with a competent clinical psychologist. Personality patterns which are slightly different or disturbing usually have some underlying basis in prior experiences or the manner in which you were raised. Exploring these in depth may be helpful in more ways than just correcting your pessimistic approach to life experiences. Good luck.

See a psychiatrist.: You sound like a pessimist but you need to check out your other symptoms of "overthinking" which maybe symptoms of obsessive compulsive personality. It might help just to talk to a mental health professional for evaluation.