I got to a point with my mum where I simply refused to listen to her abuse. If she became abusive on the phone I'd put it down. If she started on me when I visited, I'd leave. I can't say that this removed all the stress from the situation because it didn't but I did feel better with those boundaries in place.
The end result of all this, and her refusal to engage with any form of care, was that she had to go into residential care sooner rather than later. She was living like a tramp in a freezing house, eating very out of date food.
The good news is that she's now medicated and much nicer to be around. I can't remember what she's on now but the dose is low and she's bright and alert. And happy to see me when I visit. 
Not exactly a happy ending but a massive improvement, for her and for me.
The last thing I will say is something you know already. Stress plays a massive part in ill health. It took me months, maybe even over a year, to start feeling a bit more normal, to start sleeping properly, for the blood pressure to stop rocketing every time the phone rang. So look after yourself. You don't have to have any involvement with your mum if you don't want to or simply can't any more.
