Intrusive Thinking.

I'll all out tell myself "no" and proceed to give my brain other thoughts to chew on. A book, music. The thoughts other people have (reading places like this or reddit).
I know if I let that thought go to full fruition, there's no turning back.

I sort them into two categories: useful & well-founded and useless & irrational. The first ones motivate me to work harder, the latter I try to suppress by telling myself there's nothing I can do about things that are out of my control.
I can't do much about imagery though. It comes without any warning.

Its OCD and you never really get use to it, you just accept it.
You accept the fact you have this annoying friend always whispering worst case scenarios to your ear and will never shut it.