L.S.
This is very hard. When my husband left, my two year old cried on and off all night. I was pregnant with our third child also. It really is heartbreaking - I totally get what you're going through.
The breakthrough with my two year old came when I learned to put his feelings into words for him. He was so angry, and he didn't even have words to express it. I said, "Do you feel like throwing things?" It changed his world! Finally he knew that someone understood how he felt.
You don't want to talk her out of her pain. She needs you to carry it for her, which means you enter it with her and be there as she processes, letting her know you hear and understand. You can start by saying simple things, like, "You miss Daddy, don't you? I know. It's hard, isn't it?" Resist the urge to figure out some good news to cheer her up. And always be honest.
You might also look for a good counselor. It's good, believe it or not, that she's expressing it. You just have to learn the skills of carrying her pain and helping her through it. Same for your older one.
My kids are doing really well now. You'll make it through this. You might find my blogs encouraging.
God bless you,
L.
http://latenightsandlatin.blogspot.com/
http://susannasapron.blogspot.com/