In this piece Seth Godin says that your negative internal voice is a permanent fixture. Nothing you can do can get it to leave you alone. But there is an answer. Surround that voice with lots of love, it's the perfect antidote.
I learned how to do this myself a number of years ago.
I would drop something, and in the instant after, as it's falling, my dark inner voice would judge me. "Idiot!" it would say. Before I learned to challenge the voice, something I never dared do as a child, it would take over. The idea would fester and bloom and become other kinds of negativity.
Instead, as soon as I regain my composure, I call up another voice, my inner adult loving voice. "I love David very much," the voice would say, firmly, almost fiercely, "and he is not an idiot, he's a very smart, good, nice person, and I want you to stop saying that about him."
It works, I'm happy to report. The dark voice is a coward. It only picks on little kids. Confronted with a powerful adult it sleeks off to hide until the next moment of weakness.
I had a teacher who showed us how to do this. There were exercises that included punishment for the dark voice. I would lock mine in a bathroom in the basement of the house I grew up in, and make him pour a bowl of spaghetti on his own head. In my imagination, as I locked the door I'd tell him to stay there and think about it a while.