A close friend of mine was no longer recognised by her grandmother who suffered from dementia, but was able to establish a new relationship with her grandmother. One afternoon, when my friend, let's call her Annie, was visiting her gran in the nursing home, she asked her if it was all right to move gran's bible in order to place a tea cup where gran could reach it.
Her grandmother replied, " Yes, dear, I can't read it any more, anyhow".
Annie asked, "Shall I read you the gospel for today, gran?" as it was a Sunday, and gran had always been a churchgoer.
Gran said Please do, and Annie read the text. Afterwards, she asked if there were other texts gran wanted to have read. There were one or two, and afterwards gran asked, "Who are you, dear?" Annie said her name, but omitted "your granddaughter" as she knew that only confused matters. To cut a long story short Annie lived on in gran's memory as "that nice young girl who reads the bible to me when she visits"
I'm not suggesting you read the bible to your mother, who for all I know may well not want it or anything else read to her, but perhaps you can find something you and she can do together, so you, when the day comes, if it does, where she no longer recognises you, are "the kind lady who does such and such" when you visit.
My aunt's dear friend who also suffered form dementia recognised my aunt right up to the end, but thought the year was 1960 something instead of 2010, so asked after my grandmother and how my aunt's day at school had gone. They were both teachers, but had taught in different schools. They could chat about schools and how my aunt's mother, who was 70 in 1960 and died in 1964 was.
I know it is difficult, but communication can be possible.