I am a retired Speech/Language Pathologist that worked with people with dementia. There is something called Validation Therapy that is very helpful.
First though, dementia robs the most recent memories first. So if they think you are your father it is because you look like them now. They remember the younger you. Usually the recall is best from when they were in their 30’s to 40’s.
Rather than trying to correct them, which is scary to them, try to empathize. Saying, “what would you say to (name) if they were here right now?” Or, “If (name) were here what do they do that you really like?” Then you can do those things-like brushing hair, holding hands, going for a walk, doing a simple task together.
If they say that their spouse would take them home, you can ask them what do they like about home.
It is difficult when they don’t remember you especially if you feel like all those years together are gone. It has nothing to do with the value of those years. It is destruction of their brain.
Talking with people that are caring for a spouse at home helps. They have told me that a very rigid schedule that is the same every day really helps. “Now we eat breakfast. Now we wash up. Now we go for a walk at the Y. Now we have coffee and visit.
I hope some of this information is comforting and helpful. As my friend said with a spouse with early onset dementia, “It’s not fair”