Just caught this: "Your son announced last week that his move is happening in just 10 days’ time? Is it really believable that from him knowing about this to actually moving is 17 days? He must have known a long time beforehand and simply not told you and the big question you might want to ask is why?"
Probably b/c they knew you would be upset. But it might be b/c he didn't know for sure until very recently, especially if there's a job issue involved (you didn't say). Regardless, I wouldn't question them about it. Obviously, the move affects you, but I wouldn't make it about you. Focus on them, the exciting new life they're going to have, and getting in those cuddles w/ GD, etc. As others have said, do your crying and questioning when they're not around.
My heart goes out to everyone here who has been separated from their AC and GC (or GGC) by distance, etc. especially if you don't get to see/talk to them that often. EllenVannon, I'm so sorry your health won't let you visit your DD and family anymore. I don't understand why they can't still visit you, but I'm so sorry that's what's happening. I hope you can still keep in touch through FaceTime, etc.
Smurf, bless you for being there for GD when she needed you! It must have been quite a shock when she decided to move to live with her mum! But I think you were very wise not to try to stop her. But now to have DS and family move on top of it all! No wonder you are heartbroken! And yet you are kind enough to take the time to empathize with others in similar situations. You seem like a great person, and I'm sure they know that and miss you.
I'm sorry that FT isn't working out so well w/ DS and family. But, in time, as they settle in more, I'm sure it will. Eventually, you may be able to work out a schedule where you do FT once a week or something like that, even if just for a few minutes. Are you "friends" w/ them on FB or any other social media? Then, at least, you may be able to follow their activities by seeing pictures, etc. I know that's not like being with or speaking to them but, IMO, better than nothing.
"I just feel so sad every time I go into the GCs’ bedroom or see a game we had such fun with..."
I know you didn't ask for advice, Smurf, so skip this if you're not interested. But perhaps you need to get rid of the old games or put them somewhere you don't see them so easily (to save for visits). And maybe it's time to change the GC's room into something else? A den? A library? You can keep a bed in there in case one of them visits, but perhaps you'd feel better if the room was repurposed, overall? Maybe not. Just a thought.
Also, I'm sorry you don't have any other family, but perhaps you could get more involved with friends or volunteer activities? Nothing will completely fill the void you are feeling, I know, but it might help a little. Hugs!