Certainly a tough spot, but you have to choose your son over the girl. She feels like family because you have a good heart and empathy for her, but it doesn’t change the fact that she has been a partner for your son during his growing years. Put yourself in your parents shoes and imagine them taking in and keeping your early boyfriends— how would the long game on that have turned out?
You let him leave the nest and she must as well. You were gracious and generous, that should be enough.