Without wishing to be rude, but you really do have to let go. My grandaughter, who lived with us from 17 after her mum (my younger DD) died, is now 21 and in Australia on her belated gap year and having a great time! We message each other often, send photos and videos, and chat whenever we can. Of course, we miss her, but I had to insist she went, her mum would have made her go!
I believe the best we can give our adult (grand)children is roots and wings. Your son and my grandaughter are flying, and that is how it’s meant to be. My girl has a place called home, and life is too short to worry about her overmuch, her 3 younger sibs are plenty to pour my love and energy into, and my other grandchildren! That’s what I think you need: something else to pour your love and energy into, and your anxieties will recede, though as we all know, they never quite fade..