Oh dear, I fear your situation exemplifies what happens to so many folk - they start caring, thinking, or sort of tacitly assuming, it will only be 'for a year or two' ....and then it just goes on, and on, and on, and on....and all the time the elderly caree gets frailer and more dependent on you, and less capable of doing anything....and yet still keeps going somehow all the same.
So you get trapped in this 'vicious circle' between them 'not dying yet' and you 'not wanting to abandon them after all these years'......
I would suggest that one way of trying to get a 'handle' on the situation, so you can make rational and reasonable decisions about what is to happen next, is to ask the really horrible and hard, but necessary question -how much time does my grandfather have left?
Now, that might have been something you've been asking from the off, and he may well, of course, have outlived expectations, both yours and the doctors, but as he's ageing, and suffering Parkinson's, I would surmise that the 'forecast' is getting more accurate. Do you think, therefore, a 'full and frank' discussion between you and his GP would help give a 'min-max' time frame for you.
I know it sounds brutal and cruel, but the bottom line is that surely, if your grandfather only has, say, another six months of his life left, you would very, very most likely make the decision to take a deep breath, and keep going, as is, for that duration. You would find that strength in you, and make that gift to him of not rearranging his care and 'see him out'.
BUT, if the GP says, look he could go on as is, and weakening more and more, for another two years or so, then I would say that 'keeping going as is' is no longer viable. The very fact you've posted here indicates that you sense you are reaching cracking point. So often new members arrive 'in a crisis' over their caring responsibilities.
So, at this point, if your g/father really does have another good year or more left, and if you saw yourself this time next year in exactly the same situation, or worse, if you make no changes to it, then I would say that it is, indeed, time to make changes. Just what they should be is tricky -
The 'ultimate change' is for him to move into residential care. That would mean that 'someone else' does everything you are doing now, and he will be looked after from getting him up and about, meals, laundry etc, and things like all his meds and doctors visits etc. You will be off the hook for all of those 'dreary' things, and be free to spend the time you have with him as 'quality time' visiting or even taking him out, or bringing him home to you for visits etc etc, whatever he is capable of. Many people here, myself included, report that when their elder caree is in residential care, their relationship with them improves enormously, and they cease to be a 'burden' or a 'problem' and we 'get the person back'.
However, residential care is a big step, both financially and for the person themselves. Some elderly people take to it far more happily than they ever thought they would, as effectively it's like being 'on holiday' in a hotel, where everhything is done for them, and they have company and entertainment. (My MIL's home, for those with dementia, has daily activities and entertainment, which of course is partly for keeping their minds and bodies going, but they also enjoy it!).
Even if you rule out residential care, it does sound like the time has come for a lot more 'professional' care for your grandfather, by way of care workers taking over some of what you do. I'm afraid it doesn't really matter if your grandfather doesn't want this (that's a very common response) - it's an essential part of 'the deal' - ie, the deal whereby YOU will go on being the 'main carer' but ONLY if you get help in! If the choice for him is not 'my grand-daughter only' but 'my grand-daughter partially, and some professional help also' versus, say 'residential care' then he may well accept care workers a lot more readily than if he thinks that he can get you to do everything still!
Wishing you a better summer, and getting the whole situation more under your control, and taking some of the crushing pressure off you to get you a better quality of life.
Kind regards, Jenny