ceejayjay, do see your GP first, to check whether your medication needs a review.
Next, try to arrange to see your grandson - but not on a whole day, looking after basis. Perhaps a couple of hours with your daughter present would be possible, every week or so?
I'm sure that you've been good and kind looking after your family and grandson all these years - yet too hard on yourself. Why? It's time to look after yourself with equal care. You deserve it.
Be very kind to yourself, every day, and prioritise things that make you feel good, such as walking your dog and reading.
Arrange a special day off every week (no need to tell your daughter that you're not working) and make that your special treat/exploring/trying out new things day.
I enjoy crafts, sightseeing, lunching out and exploring garden centres - either alone or with a friend.