A horrid thing to say, but about six months in to this escalating behaviour she passed away in sleep, in her favourite armchair from a heart attack. I was devastated, but equally so relieved that she'd escaped it
I don't think it's a horrible thing to say at all, @EnterNight. My DM was so anxious about her (perfectly healthy) finances, she was perpetually distressed and would ring me daily, sometimes more than once, really upset about money and afraid that she wouldn't be able to pay the rent and that the council would evict her. She had no quality of life.
When I accompanied her to the doctor so he could sign the paperwork for power of attorney, she asked how long she had left to live. He told her that she was physically very fit and could go on another 20 years. My heart sank and my first thought was "I can't cope with another 20 years of this".
Mercifully for both of us, she died within 6 months, of something entirely unconnected (pulmonary embolism arising from a DVT).
It's a bloody cruel disease. For the family, it's like being bereaved twice. First you grieve for the person they used to be (a very intelligent, capable, independent woman, in my DM's case) and then again when you lose them for ever.