Dementia tends to strip civilising influences from folk and emphasise their true nature. So if you're very lucky a kind, gentle personality can become gently bewildered. But most folk are on a spectrum of niceness, and if they could be rude and manipulative beforehand, they may be far, far worse after.
But all dementias can be made worse by carers who don't adjust their whole way of thinking and acting to become dementia-friendly. It is hard. For many it is impossible, and there's no shame in admitting defeat, because dementia can be infinitely cruel and we have finite energy, patience, and time.
You are still early in learning to adjust. The compassionate communication guide can accelerate that adjustment, but it's still a tough guide to live by even if you care deeply for the person struggling with dementia.
It may not seem like it, but your son's nan is struggling too, and her vitriol is a measure of how hard she's finding it to work out what's going on in a world where nothing makes sense any more.
It took me a long time to get used to the fact my mother didn't hate me, even on the days she knew me properly. Her rage and frustrating behaviour was just her tough, determined, stubborn core trying to survive. Those qualities helped her survive life as a self-employed single parent in the sexist 60s & 70s, but after dementia developed they just made her... well, bloody hard work every single minute of every single day. She still can be... which is why I'm awake now! But my life got significantly easier after I learned that my mother was always right. Always. ALWAYS.
You have to enter their world, see it through their warped gaze. Agree with them about everything. If you're lucky they'll start to see you as someone who understands their problems and they'll allow you to lead them through the dementia fog. Sometimes anyway. Often you have to chase after them, deeper into their nightmares, and they think you're one of the monsters.
But it's hard. It's bloody hard. I rely on reserves of empathy and compassion to get me past the bad times, but even after five years of being mum's guide, and even though she's a lot calmer than she was, I still get it spectacularly wrong far too often and struggle to face the next day knowing that I've briefly become one of the monsters in her fog.
This is caring for someone with dementia. And this is why most, sooner or later, have to hand over that care to professionals who get to go home and have a break after a shift at the dementia coalface.
Get help. Get it soon. You are struggling and dementia can smell struggle a mile off. It will find your weakness, gnaw away at it, and leave you raw and in perpetual pain. Which is when we tend to come here to share our troubles and to draw a little strength from each other and our experiences.
We can't tell you exactly how best to care for your son's nan; all dementias are similar but unique. But we can tell you when you're struggling, because we've all been there. (The clue, I suppose, is in the thread title.
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Good luck.