Wow, I'm so sorry to hear you are having such a bad time, Susie. *hugs*
My Grandma is in the very early stages of dementia and nowhere near as bad as your Mum, but certain little things are starting to happen that never happened before (loss of balance, short term memory loss, not understanding what is wrong with her) and it can make it very hard to be around Grandma sometimes.
Yesterday, I just felt like I wanted to cry all day but it just wouldn't come out, but when I saw that my Mum was upset later on in the evening, I started crying and didn't stop for over two hours.
Take care and there are loads of lovely people here that will talk to you and help, me included.