It is really important that you draw the lines where you want them to be. Not wanting to look after your GD is nothing to do with not caring/not loving etc. It is entirely reasonable that you want to build a happy relationship with your granddaughter over the years looking after her twice weekly. Other than that, I assume you would happily do extra in a real emergency. But for the rest it is also entirely reasonable that you want your own free time, other things in your life etc.
I think you need to be clear and say that you love the 2 days regular childcare but it is tiring and can't manage extra unless it is an exceptional emergency. And stick to it.
Lighthearted - How long do you display Birthday Cards?
He has done it! The toolmakers son has resigned!
