I'm sure that this is a common experience for people in situations similar to mine. I've actually lived alone at various points in my life and mostly I've found it O.K. My current situation is not remotely like that, because I'm not alone. But for great long stretches of the day I might as well be, since S has at least one foot in a parallel universe where she often uses my name and appears to be talking to me but is actually talking to someone else. Very often I hear her, during her conversations with the figments of her imagination, addressing someone else as 'Love' and using the exact tone of voice that she used to use to me, and still does occasionally in brief lucid moments.
To S, for much of the time, I might as well be part of the furniture. She knows who I am and, fortunately for me, will respond to me briefly when I need to get her to do something, like eat enough so that she doesn't lose any more weight. (One positive note in an otherwise rather bleak post - she's started to put on weight.) These interactions sometimes lead to tense and tearful conversations, on both sides, but she clearly understands and believes me when I tell her I'm trying to help and we nearly always get whatever job it is done in the end because of her essentially helpful nature. So I'm lucky in that respect. But she shows hardly any interest in what I'm feeling, thinking, doing or saying (but still does, very occasionally) and that is what leaves me feeling a kind of loneliness that is almost indescribable.
I have great and invaluable support from family and friends but nothing can replace or compensate for what I have already lost and what I continue to lose day by day. I never forget though that S has lost, and will lose, a whole lot more. Despite what you sometimes hear from other people, I'm actually the lucky one. I try not forget that when I'm almost at my wit's end.