Sarah, I don't know all the details of your and your mum's situation but your thread struck a nerve with me. My mother was admitted to hospital after being found outdoors in the cold, disoriented, and injured. From hospital she went to a care home. This was in February. I knew she was having issues but never really suspected dementia/Alzheimer's until the hospital stay.
I think it's very difficult not to have some feelings of guilt. I know I lose sleep at night thinking about what I did and did not do and how that could have changed things. In the light of day, I know that I didn't make my mother get this dreadful disease. At night, I worry about her having to leave her home of over 20 years and the city she lived in her entire adult life and what a cruel and awful daughter I must be (as she constantly tells me). Again, I know that she is now somewhere safe, with proper meals and getting the right medication. No worrying about her getting behind the wheel of her car and killing herself or someone else. No worrying about her overdosing on a medication or missing a dose of something important. No worrying about her starting a fire in her building and killing the children upstairs. I know I've done the right thing, but I still feel upset, worried, and guilty about it at times. Is that how you feel? I have a lot of people patiently telling me over and over that I've done the right thing for my mother AND for myself. I don't always believe them in my heart, even though I know in my head that they are correct.
I also think it's difficult when the disease takes over and our mothers tell us things that aren't true. My mother will tell me she was fine where she lived and had all sorts of friends and the truth is, she was isolating herself and would go for days without any sort of contact, didn't remember to eat, and couldn't care for herself or her home. Now she is busy and has company at least eight hours of every day, she is clean and safe, she has clean clothing and proper nutritious meals three times a day (and all the ice cream she can eat!), yet she will call me and say, I haven't seen anyone all day, i haven't eaten in days, they don't give me my medicine...and of course that's just not so. It's hard. It's really, really hard.
It sounds like you've had a difficult relationship with your mother which surely makes it all that much worse for you. The advice I get is to be kind to myself, so I will tell it to you.
I don't know a lot yet about how this disease works, but I am certain that YOU are not to blame. Maybe when you have the thoughts about what you could have done differently, you might try telling yourself what you told us: you did the best you could under difficult circumstances, and your mother is now safe. That counts for a lot.
Hope you can find some comfort or peace, or at least less turmoil, for yourself. Please hang in there.