I want to cry

[Phteven]

It's just brutal when they are being unrelentingly awful.

In the short term I find that taking the whole show outdoors can help. Not to the shops but to a park or for a swim or a bush walk. It gives me some breathing space and the ratty one something else to focus on.

DS2 was so awful last year after we moved that we ended up going to a pediatrician and a psychologist. It has helped so much.

[onthefly]

I just wanted to add that it's not you. There is something going on with your kid. He's that lovely, wonderful boy, but there's something (or things) happening that he doesn't know how to deal with.
My son is like this too. We've also sought professional advice. He's been diagnosed as being on the autistic spectrum and sensory processing disorder... he's seeing a pych for anxiety. We are making progress. He's slowly learning to self manage, and though he still gets angry and violent, it is getting a bit better overall.
Getting out of the rut - go outside, add water, something different - does help in the moment. We can easily get stuck in the habit of fighting with our kids. It is hard, though, when they are so unrelenting like this.