My Dad suffered from Alzheimers and progressive onset Dementia and it's a hell of a struggle for the sufferer and lest we forget the person who's most responsible for their care. My Mum is herself elderly and found herself in situations you wouldn't wish on anyone on quite a regular basis, you know?
Anyway, for an insight, I'd recommend keeping an eye out for the Terry Pratchett doccos on BBC4 cos they'll be shown again no doubt.
I can empathise with the sadness of realising in your moments of clarity that you're losing your mental faculties - luckily my Dad was never one to take himself too seriously, and he laughed at himself as much as feeling sorry for himself, if not more. But it's hard. There's a moment in that Terry Pratchett documentary when he's giving a public reading from his new book, and he becomes unable to read the text in front of his eyes, announcing to the thousand odd people in front of him that he can't make out the words for 'the shadows'. Very sad, you know?