Well done for refusing to be POA. It's the only sensible decision, so stick to it.
I had a verbally abusive mother too, but, over time, I distanced myself and took little notice of her ridiculous comments. They were her problem, not mine. She didn't like it when I laughed - so try laughing.
Contact was short and sweet, in the company of others - never alone (when she'd be most vicious) and, ideally, on neutral ground (lunch out, for instance) where, being in the public eye, she retained some manners.
You don't 'owe' her, just for being your mother. You may feel obliged to help out, though. Do it on your own terms, not every weekend, only when necessary. Start to find out about sources of help available to her - and plan to take a back seat in future. First and foremost, look after yourself.