"What do you do when you have some negative thoughts in your head, or overthinking - what is the simplest thing one can do?"

Dalai Lama : Negative emotions are mainly related to two things.

One, self-centered attitude. I, I, I, my, my, my, like that.

Then, another sort of factor is we accept reality as it appears.

Nothing exists as it appears.

So since all negative emotions, are very much based on appearances, once you realize nothing exists as it appears, the wisdom side, and altruism is the antidote to self-centered attitude.

So self-centeredness is one factor. Then the belief, grasping at things as they appear - these two things are the basis of negative destructive emotions.