I feel shut out

Hello fellow grannys. This is my first post as I’m looking for some advice from you lovely ladies. My oldest son and his partner gave me my my first grandson 3 Years ago. My granddaughter followed shortly after and I feel completely shut out from their lives. They live 3 hours away so I rarely get to see them. The first year after my grandson was born we only got to see them once a month. We either went to stay with them or they came to ours for the weekend however this has dwindled down to about 4 times a year. I split with my husband about 2 years ago and this was an extremely difficult time for me but I don’t feel like I got any support from my son or his partner. Just spending time with my grandchildren would have brought me so much joy during this very stressful time but I feel like I have to beg to be allowed to stay with them. When I do stay I am only permitted to stay for one night which just isn’t enough time.
I was so very excited when I found out that I was finally going to be a granny but I feel rather deflated about the relationship I now have with my grands. The atmosphere is so tense when I visit and my sons partner practically ignores my presence. I have tried so hard to be helpful and get involved but she just won’t let me be a proper granny. I have never bathed the children or changed a single nappy. She barely lets the children out of her sight. I recall one occasion when I tried to take the baby out into the garden and I was told to bring him back. I just can’t make any sense if it. Where am I going wrong? Does anyone have any advice as I am truly a loss?