I believe there is a direct relationship between how much we love someone and the time it will take to grieve if we loose them, so for me not much hope in that direction
That is certainly true, and I've been grieving for 15 years now with no faint idea of how many more years will be involved. However, the course of the disease and its effects, the time involved, the wear on the person caring... all these things lead either to a breakdown, a totally solitary and exhausted, miserable life, among other things.
I thought as you express yourself. Nobody had the relationship that Jan and I had/have. But over time, she has lost more and more faculties, and while I still visit most days, I have reduced the duration of my visits as she 'loses' me [if she has even registered me] after 30-40 minutes.
I found that originally, I stayed longer then realised that was more about me than her. I
needed to be with her, to share in some way her torment. She would get agitated if I stayed too long.
We've been together 40 years now, 37 married. You don't - can't - write that off. Neither can you dust off your hands and say "that's it then, she's no longer the person I knew, so I'll stop and make a new life".
All you can do is what your heart and circumstance tell you. My heart tells me to keep on visiting until the end. My senses tell me that Jan does not gain if I stay longer than she may want, or if I centre too much of my days to doing that - 50 miles a day of driving to the home is wearing in winter time.
And dangerous. I took the vicar to see her last week and just managed to miss a white van that came around a corner on a narrow road, on my side. Missed another one a couple of days later - he was again coming around a corner in the middle of the road, but was steering one-handed as he was on his mobile at the time.
Hearts are much bigger than we give them credit for. It isn't impossible to make a life yet more complex by unexpectedly finding someone else - and that does not mean that one loves the person with dementia less than one did - or less than someone else who has not been on the path so long, or whose loved one is not yet at a certain stage, or who has made a decision to devote the rest of their time to them.
There is so much hurt in this dementia business. Time does not heal, but it changes things, and that is something we have no control over.