I was exactly in your position - only child, mum with early dementia. I had the constant phone calls for reassurance, the medication, appointments, shopping, meals. It was all me, all of the time. The thing that broke me was the phone calls because even when I wasn't there I was still part of it. However much you do it's not enough and you can't have a day off ever because there's no cover.
Someone earlier mentioned a catastrophe, that's what it took for me to hand over responsibility to someone else. Mum had two weeks in hospital after a fall and came out with a care package. That didn't go so well mostly because she couldn't recognise her home of sixty years and she's currently in residential care, I don't know whether she'll recover enough to go home but if she does, it will be on different terms. It's been a revelation, I can do what I want, when I want without worrying about the impact on mum or how many phone messages I'll return to. I hadn't realised just how much I was doing until I stopped doing it.
There are some jobs that can easily be done by someone else (personal care, meals, giving medication, cleaning, gardening) leaving you to do bills, banking and fun stuff. There's no time for fun stuff when you have everything else to do. Imagine what would need doing if you were on holiday for a fortnight, what would need to wait for you to come back and what could be done by someone else.