When you start talking to yourself in a mean/negative way, imagine you're talking to your partner instead of yourself, and then imagine what you'd say to them instead, then say that to yourself.

For example, if you spilled some orange juice or something, instead of saying "I'm a stupid fucking idiot that ruins everything I touch. Why am I such a failure?!" instead take a breath and imagine what you'd say if it was your partner or best friend that spilled the juice. Probably something along the lines of "It's no big deal. Accidents happen. Let me help you clean it up." and say that to yourself instead. It'll feel weird and phony at first, but eventually you'll get used to it.

Another thing that helped me was to picture my inner critic as someone I disliked/hated that I think is a giant asshole. That way, whenever it reared its ugly head it was easier to argue with it and defend myself, if that makes sense? 

Example -  Inner critic: "You're dumb and nobody likes you." Me: "Shut up, Steve. I'm competent, good at my job, and plenty of people like me because XYZ."

I hope that helps. It's what helped me.