PoppyandGrandma, I've been on both sides of this coin. I was the daughter who moved overseas many years ago, with my husband and young children, leaving behind my devastated parents. I'm an only child, and my children were my parents only grandchildren. It was sad for us all, and we didn't make the decision lightly. We did it for better career prospects for my husband, and what we hoped would be a better life for the whole family. My parents were absolutely gutted, but they gave us their blessing to go, knowing that we were trying to make a better life for our little family. They visited us for many wonderful holidays over the years, and they were happy that we had obviously made the right decision, especially for our children, who had a better education, lots of freedom, and so many experiences that they otherwise would never have had. We kept in touch constantly, and visited each other as often as we could. The close bond that my parents had with their grandchildren was never broken, and was always strong.
Fast forward to the present day, and I am now the grandparent whose son, DiL, and my two grandchildren (one whom I've not yet met due to Covid!), live another country. The longing to see them never goes away, but just knowing that they are happy, and enjoying their life, having great experiences, and making wonderful memories, is one of the best possible things that we can hope for.
I promise that the pain and upset you are feeling right now will ease over time. And when you get to visit them, and see the life they have built for themselves, you'll understand that it was the right decision, and you'll be happy for them.