I think getting used to the idea that there will be no grandchildren is very difficult, a bit like knowing you cannot have children. I have a friend in this position, her daughter died in her 20s and her son didn't marry until in his 50s and his wife is the same age as he is. She has said little, but I am aware what a great unhappiness this is for her.
I think your position is made worse because you are in a group that no-one thinks of, so it is never discussed, each person left alone with their grief.
I do not know what to suggest. Madgran has made one suggestion, counselling might help. Finding others in your situation might help.
Either way, if it does not rub it in, stay with us.