It is absolutely OK to say no more, I've had enough, I need to have time for myself now. I will happily go on the occasional day trip, and pay for it, but that's all. But one word of caution from my own experience, it may be taken by her that you don't want to be with them and that makes you an unnatural mother/grannie.
I too, think you know what to do but wanted affirmation that it was all right for you to stick up for yourself, wanted us to say you were right to do it.
You say you have done it all these years because that's what grandparents are supposed to do? Really? Well, no wonder I don't have any relationship with my own, because I simply won't do something I don't want to do, not at my age. I don't like people much, never mind children, and am selfish about my time, plus there are serious health issues to contend with, none of which is relevant to this post, but I so hate it when expectations on how you behave are forced on people. We are not all the same, because we don't like being around children/people in the family, doesn't mean we don't care, which is how it came across to my son's partner. But any excuse to kick the old dragon out of their lives, one excuse in a long list....
Be true to yourself.