Imm6 There is a big difference between caring for a grandchild and caring for an elderly person. A grandchild is growing up and is less and less dependent on you as he gets older. An elderly person is the reverse, as they get older they get more dependent.
With your grandchild, you could see ahead, to days when as much as you do things for him, you also have plenty of time on your own, and to plan outings and a more free life. With your mother, even though carers do the hard work, you are being more and more tied as she is less and less safe to be left alone and totally dependent on you for company.
Of course you feel as you do. I think you need to talk to your GP and your mother's social workers and make it clear that you need respite. As others have said. Could she go to a care centre, can you afford to pay for day care at a care home. Some do offer this service. Can your daughter, repy all you have done for her and her child by looking after her for one or two afternoon sor days a week to give you some freedom?