(((jak))) I wish there was some magic that could take the pain away, but I guess there is just stupid sayings like 'it takes time' and 'things will get easier'.
Just do things one day at a time. Start small, like Jo says, and do one thing for yourself today. Focus on you. Is it possible for you to get another pet? One that doesn't require as much care, but could be your focal point for a while? Or maybe go down to your local animal shelter and volunteer to walk the dogs there on your days off. They usually always need some sort of help.
Don't worry about her coming back right now. She probably isn't going to today, maybe not tomorrow, but beyond that, who knows? Don't worry about it! Do what you can for YOU today and keep doing that tomorrow.
And smiling may not feel natural yet, but it will. Tell yourself to smile, even when you don't feel like it, eventually you might find yourself smiling automatically. Here's a little one from me to you Oops, that was a big smile!
Oops, that was a big smile! 
