Oh Cathy I just want to send you some more absolutely HUGE hugs.
I have almost 5yr old and almost 7yr old girls, and separated from their dad at the end of 2008 (now divorced). So I feel like you are at the stage I was back then (except it was Em who was recovering from surgery and not me)...
All I can really say is that you have been given great advice here, and to reassure you that this time will pass, and life will definitely improve. I think its pretty clear too when you read that each of us here who have been through it assure you life gets better - its a very tough time at the moment, but the rewards will be worth it.
As hard as it can be, take any help you can get from friends and family and do remember to look after yourself as well as you can.
I'll definitely be thinking of you and please let us know if there is anything we can do.