I think you know YABitU but I do understand. I'm in a similar position with my DM, and it is exhausting physicallly and emotionally, for everyone concerned. But, I was also expected to take her to all her appointments, despite the fact that I live 2 hours away, run my own business and have a young family, where as my DBs live 5 minutes away, both have accommodating jobs and grown up children.
I took a step back a couple of months ago - in fact I rather lost the plot and said "no more" and that DBs would have to step up. Which they have done. It turned out that she wouldn't ask them as it was a daughter's "duty" to look after her DM at ALL times.
She had also convinced herself that she couldn't do anything, when in actual fact the doctor's had recommended that she get off the sofa and be gently active. She is actually better mentally, and physically now.
And, the phone conversations can be exhausting. Whilst I still listen on the phone, I don't let it get me down or wind me up. I just agree with everything she says, or ask what she thinks is best for her. This seems to work.
Sometimes, you just have to stand up and say "no", although you will get a kicking on here for it. But, if you can't rant here then where can you?
So, you have my understanding and sympathy, but it won't stop you feeling guilty.