I have a friend who works in mental health and it's terrifying how the mind can dominate the body. Someone can be perfectly healthy but if they suffer a mental illness, they're just completely crippled and unable to live in society. (I should point out he works with patients prone to violence, I realise not all mental health issues are anything like that severe.)
Both my grandfathers had dementia, it's a really horrible thing to see. Perhaps worse than the effect on the individual themselves is the effect it can have on their partners.
I hope you can at least take comfort in the fact she is at peace.