Oh, I so understand what you are going through. My mum had dementia and towards the end she was in the same state (I use that word deliberately) as yours. I was particularly repulsed (again, deliberate use of that word) by the state of my mum's teeth and the way she gripped the spoon rightly with them. It's hard; it's so hard to keep visiting. Is your mum being well looked after? If so, please do give yourself a break. I did that for a while, explained myself to the lovely staff who took very good care of her and they understood and were very kind. The other thing I did for a while was to go in when I knew she was usually asleep just for a little while. I'd chat with the staff to make sure she was ok, sit for a while, knit and/or read and stroke her hand. No interaction, because she was asleep, but also no screaming, or staring wildly and no views of teeth. A cop-out? Yes, maybe.
It's a rubbish illness to see and incredibly difficult for families as well as the person themself, of course. My brother didn't visit mum very often at all - he couldn't cope with her taking all her clothes off. I understand that and he was absolutely brilliant at dealing with admin stuff e.g. Power of Attorney and so on, and looking after her house. We all do what we can, when we can. There isn't one answer which suits everyone.
Another suggestion is to google Alzheimer's Society. Their Talking Point forum is fantastic - no reflection on Gransnet, which is also fantastic, but everyone on there has direct experience of this dreadful illness. I had some wonderful support, sometimes in the wee small hours when things always seem awful (well, they are) and you can say anything and be unjudged (Yes, I know Gransnetters don't judge, but still ...). I can't recommend it too highly.
Sending thoughts, hugs and understanding. xx
PS I know this rambles on a bit - sorry, but it's something I feel very strongly about. I now volunteer with the Alzheimer's Society, which has given me some insight into it, from a safe distance.