I'm sorry for the loss of your old dog, OP, and apologies if this thread is now done, but I have a very similar problem with my old dog and am looking for advice.
I have a 14.5 year old cocker spaniel, who I have had since a puppy. She is profoundly deaf (and has been for quite a while) and has very little sight. The last time her eyes were checked, the vet told us she is completely blind. We believe that she can see a small amount, probably at a distance, but it does appear to be quite random and it is getting worse at the moment. Over the last few weeks, she has started to walk into things, sometimes with a real bang.
She also has dementia and many of the things that others have mentioned on this thread are familiar to me with my dog. I can't walk her off lead on my own now, as she suddenly panics, thinking she is lost, even when I am right next to her, and runs like the clappers in any direction. I can't catch her as she is very fast. She paces relentlessly in the house. Oddly this is a real problem immediately after we have returned from a walk and it is much worse when I am out of the house as she runs about the ground floor of the house looking for me.
She is also obsessed with food, when she hasn't been very interested in the past. There are times when she goes to check her bowl every few minutes, even when she has just been fed. She takes her food to other parts of the kitchen to eat, leaving a trail of debris everywhere.
She whines quite a lot, particularly after eating. She has been doing this for quite a while but when she has been checked over by the vet, nothing untoward has been found. She licks her paws quite a lot too.
I feel that her life must be miserable, but she appears to love her walks and she likes being with me, wherever I am. She follows me everywhere (I am working mostly at home at the moment) and, because she needs to be as close to me as possible, she is always under my feet and I have almost fallen over her on a number of occasions. I have also trodden on her paws, as she moves so quickly.
Up until a couple of weeks ago, she was soiling regularly in the house, sometimes several times a day. Oddly, that has completely stopped for a week or so for no reason that I can see .
My vet has told me that any time from now would be the right time to have her PTS. She has based this on the fact that she is bumping into things and that, as lock down ends, members of the family who have been around for her, will no longer be around so much. That does include me, and my previous arrangements with dog walkers are not going to work anymore. We have a bit of a reprieve, as one of my DC, who should be at university, is now going to remain at home until September.
But I'm really struggling with making the decision. Physically, other than her sight and hearing, she appears to be fit as a fiddle. She is slim and runs like a puppy.
And this is the horrible part. I have to accept that she is making our lives pretty miserable and dealing with her is increasing the stress that we have been under lately (for obvious reasons). I know that I have had enough (and my husband certainly has!) but that makes the decision much, much worse. She has been a very sweet dog and I know I shall miss her, but not as she is now.
I should be grateful for your thoughts. Please be kind as I am struggling with this as it is.