An old friend of mine had her first child at 15. 14 years later her daughter presented them with their first grandchild, my friend was only 29 and a grandmother. She went on to have two more children, one of them born on the same day as her second grandchild. The closeness of all their ages and their youth made them a very close, supportive family. The young grandparents had the youth and energy to enjoy having babies and small children around, which many older GPs don't.
Personally I think your negative attitude. and particularly that of your husbands. is ridiculous. There is no reason at all why you should not have more children of your own. You are your own family, what your son and his GF choose to do is irrelevant. If you continue with your current attitude you will alienate your son and his GF and worse still miss out on the joys of being a grandparent, and a parent again yourself. Your grandchild, and your own future child(ren), will just accept their extended family for what it is - hopefully a happy family.
In past generations it was very common for women to still be having their own children after they had become grandparents. Why should it matter now?
Life takes unexpected twists and turns and doesn't always follow the path we had envisaged or planned. Embrace the changes, look at them a different way. You're going to be grandparents! Young, youthful, energetic, full of life and enthusiasm, and with time to see your grandchild become an adult, too many older grandparents don't have that privilege. Stop complaining that your life is being turned upside down and not going how you expected it to and just get on with it. Your son and GF need your support now (not financially or materially) just to know that you're on their side and are there for them, and most importantly for you grandchild.