Every sympathy. Focusing on the positive is very good advice, but it's not always easy, is it?
I am lucky enough to have one GC, but no more children are possible for my daughter and family because of SIL's now critical illness, and they all live a long way away. One of my best friends of 50 years now has all three of her ACs living near; and a total of 6 grandchildren. Hard not to envy her!
But another friend's son died very suddenly in his teens, her husband left her soon afterwards, taking all their money, and her daughter is now middle aged and single with no children. I admire this friend so much. She has rebuilt her life, is much involved with her local community, and is clearly much valued by very many people. I'm sure you are too. You sound as though you have so much to offer.