I have every sympathy for the OP. Of course it was the wrong response but we are only human. Shock and pain make us blurt out our deepest feelings sometimes, instead of the measured response we know we ought to give.
Our eldest DD and her husband emigrated to the US in August 2014. That means we haven't seen them or their 4 kids (except on Skype, of course) for more than two years. I miss them more than I can say. It is a sadness that never goes away, but you learn to live with it.
In some ways, the months prior to their leaving were worse than when they had actually gone. At first, it looked as if they were going to Europe, as our SiL had been offered jobs in Sweden and Germany. That would have been bad enough, but then he got a very good offer from the US. Throughout all this, and the long process of getting visas, I had to maintain a smiling face, listen to their problems and offer encouraging remarks. All the time, my heart was breaking. I used to turn to my other DD for comfort, which was hard on her but I had to have a shoulder to cry on. My DH wasn't much use, as he just said "Well, they're going. Nothing we can do about it."
We were on holiday when she rang to said they'd got their visas. I congratulated her, rang off and burst into tears, right there in the middle of a seaside crowd. At first, there was some talk of them coming home after three years, but I knew they never would. Now they have bought a house, they have their green cards and they have a whole new life out there. It is a good life and they are all very happy, including the kids. They know we miss them and they miss us, but for them it is an adventure. It's much harder for those left behind.
Bear up, Gardenman99. It hurts, it's awful, but you've got to support them. If you don't, you'll lose them completely. I'm sure they understand that you are hurting but, if they really want to go, they're going anyway. Plaster on a fake smile, be as helpful as you can (we ended up having to clear and sell their house!) and start saving for a visit. We're hoping - fingers crossed! - to go next year.
Also, I found that large spoonfuls of peanut butter were very comforting at times when I was feeling very low...