I am way tooooo familiar with Alzheimer's. Been through it with both my mother-in-law and my stepmother. MIL died two years ago and SM is now in an Alzheimer's Care facility and likely will not live much longer. And, now my Dad is showing some signs of dementia although his doctor thinks his is due to occasional oxygen deprivation because of Dad's lung disease rather than to Alzheimer's.
I know exactly what you mean about dealing with repetitive conversational memes....drives you CRAZY! Like having one line of a song stuck in your head. It helped me some to realize that MIL and SM were NOT doing it deliberately just to drive me up the wall.
Alzheimer's patients seem to get "stuck" in a particular moment and loop round and round through it like that movie "Groundhog Day"... except that they literally have absolutely no recollection of having said exactly the same thing two minutes previously! Everybody ELSE in the "movie" suffers the repetitions while they themselves are blithely unaware.
I suspect your Mother's pet phrase is her way of saying that she is aware that something is not quite right with her, even though she can't explain what it is that is wrong. And, that is really sad when you think about it.
My MIL used to constantly say "I don't know what is wrong with me, up until 2 weeks ago, I had a GREAT memory but now I can't seem to remember anything." And she said it what seemed like 10,000 times every day for a couple of YEARS at least. Then, all of a sudden, she just completely stopped saying that phrase and we realized that she had deteriorated to the point where she no longer realized that anything was wrong with her.
Similarly, for years, SM's favorite phrase was "seems like I'm supposed to be doing something...?" spoken with a questioning lilt that made it clear that she was hoping somebody could/would remind her of what it was that she was "supposed" to be doing.
One thing I did was with both of them was, when I'd start feeling like I was ready to scream, I'd challenge MYSELF to find something new to say to every reiteration of their buzz phrases. That made it something of a game for me and I'd sometimes find myself actually waiting rather impatiently for the next repetition because I'd just thought of a really good reply and wanted to see how it would go over.
Second thing was that I started trying to make my responses something that suggested that MIL and SM go DO something. Eg, with SM I'd say something like "Yes, you're supposed to be helping me fold this big stack of laundry." or "I think you were going to sort through this pile of socks and match them up." "You said you were going to vacuum this afternoon." Getting them busy with some sort of task seemed to short-circuit the cycling, at least for as long as they could manage to stay busy on the task.
With your mother, you might try responses like "How about drinking a cup of coffee? that might help you wake up." or, "You probably just need to go wash your face." Or "I bet a nice walk around the yard would help" or, "well of course, you can't wake up! it's bedtime, go get you in your nightgown on"... whatever is appropriate to the time of day and what you would like for her to be doing other than driving YOU bananas!
Fortunately Dad has not started being repetitive. Instead he has some weird "echo effect" thing going on in his brain where he'll see something and have such an overwhelming sense of deja vu that he honestly thinks that same event has happened numerous times in the past. It seems to happen most often when we're out driving somewhere. Eg., we'll drive past someone getting a ticket and he'll say "I see that same guy in that beat up old chevy van getting a ticket every single time we drive by here." I used to argue with him that, as far as I could recall, he and I had never driven past this particular spot together before. Now I just say something like "yeah, you would THINK that guy would learn better than to speed."
The human brain is a weird, weird thing!
As for ketogenic diets, I had never heard of such a thing when dealing with MIL. But, it definitely helped with SM although, except for short periods of time, I was not in control of her diet. And now, when I can keep Dad from bingeing on carbs (he loves to bake) I think he is sharper and suffers fewer of the deja vu episodes.
How much a ketogenic diet will help seems to be highly individualized and probably depends, in part, on how far along the Alzheimer's has progressed when you start the diet. And since, as patients progress more deeply into the disease, they may actually become EASIER to care for because more amenable to suggestion and less agitated by their own knowledge of their disease, it is certainly possible that "improvement" for the patient may make it harder for the caregiver.
Whatever you do, don't beat yourself up for your FEELINGS. Dealing with Alzheimer's 24/7 is unbelievably difficult. It doesn't LOOK that hard to someone who only deals with the Alz patient for short periods of time, but the hour after hour after hour, day after day after day of being constantly "on alert" while dealing with the utterly mundane just wears you down mentally, physically and emotionally.
Make sure you get some help! Yes, having someone that you can talk to who "understands" is helpful. But in addition to an understanding ear, you NEED at least one full day per week (and I mean a full 24 hours) that you are NOT "on duty." And, if your Mother is a night-time wanderer (as more than half of Alz patients are) you NEED someone else to stay with y'all at night so that YOU can sleep uninterrupted. Do NOT break your own health while caring for your Mother.