I think the OP is sad because she had her daughter and Grandchild live with her since birth.
It could be she is missing them because the house is so quiet without them.
They are only across the other side of town, you will see them often, but in the mean time you need to think of doing other things, if you don't work, get out and see friends or do something you enjoy.
I don't think it is helpful for posters to say their children live the other side of the world, that is not your reality, and as for pull yourself together, it is wrong to say that. It will take time to get over these feelings but only you can do it. My personnel feeling is you need time to get used to it, and these feelings will pass.