Susieboxer, I have a daughter who lives this same distance away with her husband and GC (my only). They live near SIL's family who see them all the time. So I really sympathise. It can be very hard.
I cope myself by trying to be thankful that my DD and GC enjoy coming for visits (which because she is lucky enough to be able to work part-time, do not have to be always with SIL).Because they always come for several days because of the distance, we can have quality time together.My GC is now old enough to remember and look forward to seeing us.And I go to visit them too for short periods occasionally - when it is convenient for DD and SIL- trying to bear in mind that they have their own lives, work, friends, interests etc.....!
And I also try when I can to remember those of my friends whose children and grandchildren are now living the other side of the world; those who do not have any grandchildren at all; and those whose adult children are disabled and will, sadly, never be able to live full and independent lives, let alone have families of their own.
Both sets of my own grandparents lived the distance you mention away from me, too, and in the fifties that took longer to travel! I would have loved to have seen more of them - but the time I did have with them was very precious; and I have wonderful memories. Conversely, a friend of mine had a grandmother who lived on the same street. She was always around - but was, apparently, not liked at all by ANY of her grandchildren!
I have thought myself about moving closer - but leaving an area where you have built an independent life and friendships over many years for somewhere where you could become over dependent on your immediate family for social contact needs very careful consideration.And if other work opportunities present themselves, or (perish the thought!) if other, more unwelcome, life events occur - there could be other moves in the pipeline for your DD.
I think in these circumstances it is very important to continue to have a life and interests outside your immediate family members. This will give you other things to think about; makes you more interesting as a person;you will have more opportunities to make more friends; and you will also have skills and enthusiasms to pass on to your grandchildren when you do see them - whether this is making/creating things, cooking, gardening, music, making up stories etc. etc....
Take care, and try to be positive.