TGB70...It's not you and you are not alone. I'm come to the conclusion many adult children who are unhappy or stressed tend to take it out on the only people who give a crap...their parent or parents.
I had this great daughter I was close to....then her husband walks out on her for a 21 year old, while my daughter was pregnant no less. They also had a 2 year old at the time. I realize now that is when my daughter's heart was ripped out and she's never been the same person. But, as much as I have cried for her, I'm done.
She took it all out on me and for the longest time I feared (and still do to an extent) she would estrange from me and I'd never see those two grandkids I love so much, especially my grandson who has always seen what has been unfair to me and he's only a kid, will turn 6 this year.
So I've learned to bite my tongue, and do what I'm told but it has indeed effected my feelings for my daughter. I still love her but in a way, she's gone and the person that replaced her does not have the same heart. She's been critical of me, even kept me away from grandkids for weeks when she was mad one time. I figure it wouldn't have been too hard for her to make it permanent, but I was lucky to get back in and I'm staying , even if I have to cheat and lie. I will not give her any excuse to ever do that again.
Sure, sometimes I feel I am being used, a lot like you feel. I not only help her with the kids but also pay for a lot of things. But grandkids are worth it. Most likely they will get most of anything we have left when we are gone.
I hope you can get back in too with your grandkids.
But 10-1, your son is not happy in his life, for whatever reason and he is taking it out on you. That's what a therapist told me about my daughter.
Sounds like you are dedicated parents and grandparents, and we are usually the ones most stepped on. I don't get it; I know others who don't care so much for their kids and even less for their grandkids, but they are the ones most wanted by both. Life is strange.