Forever - not read the rest of the thread yet, but just wanted to tell you I feel for you and completely understand. My exh left me and 2dcs, then newborn and 2 years old, nearly 3 years ago, just after Christmas. The first Christmas, he insisted we would have to split it and I would just have to get used to it, and I hated him for it. Like you, I sat and sobbed. I thought the DCs should always at least wake up in their own beds on Christmas morning and we could share lunches perhaps. But like me, you will have to accept that "their own beds" and "their own home" is now also at their dad's house.
In a way I am glad he insisted it from the first Christmas as I have never got used to having them to myself. It is not easy, but you can always make the most of child free time and then the time you have them becomes extra special.
Some people do alternate Christmases but I think while they are so little, neither of us could bear the thought of a Christmas Day with no kids, so we split across the day. One has Christmas Eve/morning, then pick up time is 11am for he other parent who then has Christmas lunch and Boxing Day. I've now done one years of each -weirdly I thought I'd hate the not having them Christmas Eve and morning the most, but in fact it was the other way around. This way I could go out with friends on Christmas Eve, see Christmas in, have a lie in (!) and then collect them and enjoy the rest of the day. Having them Christmas morning just seemed a rush if presents and then off they went.
I know some people prefer alternate Christmases or may need them due to distance (wee are about half hr drive apart which works ok) but if you can do it this system does work.
Good luck, go with it and try to occupy yourself with other things when you don't have DCs - have some grown up christmas celebrations which will make the children ones all the more special. Xx