Do you have siblings or are you the only child? I have some understanding of your situation, but from the other side; my mum has dementia (and dad is her main carer), and one of my sisters (I have 2 other siblings) lives in the USA.
The three of us who are here do as much as we can to help, within the limits of our own lives and families. My sister in the USA only visits once a year or less, so clearly cannot do much to help in practical terms. I feel ok about this; she made her decision to live there years ago, whilst mum was still young and fit, and obviously none of us could foresee mum's dementia and what a toll it would take on all of us.
I wouldn't expect my sister to move back; her life and family is there, and I accept that, and I think the rest of my family does too. However, she is really supportive of dad as far as she can be from afar, and is always ready to listen whenever I want to call her and sound off about the awfulness of it all.
I don't think you should feel guilty; your life is here. Your mother might be saying now that she'd like you to be closer to her, but it sounds as if she wouldn't have said that before her dementia came on, when she was obviously more rational than she is now. I know my mum, if asked, would say she'd like my sister to live close by, but she'd never have said that before her dementia; she knew that my sister's life was in the USA. I'm sure you do a lot in terms of being supportive of other family members who are caring for your mum, and are perhaps able to visit reasonably often.
One thing I've learnt in my journey with mum's dementia is that you can always feel guilty. Guilty about not spending enough time, guilty that your other elderly parent is shouldering the bulk of the burden, guilty about getting irritated with the person with dementia, guilty that you find looking after that person boring and frustrating.... In the end, I've just come to accept that whilst I might feel guilty sometimes, most of the time, I'm doing as much as I can within my own limits, and that's all anyone can ask of us. Don't feel bad, and remember that you have your own life to live as well.