Yes you are, but I think probably you are happy enough to do it generally.
The other grandparents could have helped out this summer. However, it's your children who are the people who are really taking advantage of your good nature. If you have always done everything for them, they won't ever consider you, unless you choose to speak up.
Why do you do 6 days a week? Can't the parents cover that 6th day?
Sooner or later your age and health will mean you just can't do as much. Then they will have to make other arrangements. What about your own social life? I assume there isn't much time for it. Is your husband happy with this life too?
As the grandchildren get older and more independent, you won't be needed and won't see them as much. Then you will have to make friends/get a social life of your own, to fill the void. You could tell them now that it's getting harder for you, and ask them to cut down the days, even by one or two, by next Spring.
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