i try very hard to turn my brain off . i have been working in the same organisation for over 30 years (doing different things) and still i think i am not good enough. I am wired like this. i hate it.
My wife has had a bit of success with mindfulness and meditation. They are both really about disengaging your excessive intrusive thoughts. It would be worth a go if you can be bothered. It's probably not for everyone but plenty of people swear by it.