It's so exhausting. I won't pretend I know exactly how you feel, but I was practically chained to my grandmother with ovarian cancer and bad lymphedema for just a week, and that was well enough for me. She was close to her death and probably had rapid-onset dementia, because she would wake me up from the other room with cries and sobs about where she was and who I was. She had a pretty bad 'tude as well, but I tried not to blame her too much, since she was literally dying. I get the backpack metaphor. One of the most painful aspects is probably the loss of a social life at that time, nothing can compare to the weight of responsibility and the burden you have to carry for g'parents who frankly treat you like shit despite you helping them, and who at the same time rid you of life opportunities through no fault of their own. It's like you're supposed to do all of the things young people should go through, live life, you feel like with every day you lose opportunities to be in the now with other people who feel and look actually alive, but instead you're here sitting with an old relic, a remnant of a person almost. You can literally feel your life stagnate and get stale in the air ith this person you used to love, but who you now resent to an extent and who's about to pass away. It's very hard on everybody.
Oh, and I'll never forget trying to give my grandmother a foot bath and rub for her really gnarly foot on her edema leg she physically couldn't care for. That was nasty.