Upsdaisy, So sorry you are feeling so rubbish. I was dumped and heartbroken but that was not when I split up with my children's father, so I can not imagine the PAIN of the two combined, plus the practicality of having two children to look after.
I can't really say anything to help, except that you should treat yourself very well, because you are very important to your family and your friends and your children. ONE person has foolishly decided to leave you but that doesn't mean that you aren't absolutely worth your weight in gold to a million other people. Don't let one person's decision to leave you define you.
I know it is so hard to believe right now, but it IS possible to be a single mother and to be happy about that. don't feel that the misery will stretch out ahead of you for the next 30 yrs. It won't be like that, honestly.
I'm so sorry that all I have to offer you is clichés, time will help. I was on anti-depressants in 1999 following a bad relationship breakdown, and I just wanted to escape from myself and my own thoughts. The same thoughts did lap after lap in my head.
I hope your children are not seriously ill. Don't worry about or feel guilty about being unhappy on your DC's birthday. He/She won't remember.
Keep posting, keep venting. take all the help you're offered. I don't know if you're the type but I found running so enormously helpful to me when I was post-break-up insanely depressed. I was also on anti-depressants mind you, but I really think that the running helped just as much. The thoughts that went through my head as I ran were like dreams I think. It processed so many thoughts that needed 'airing'. If somebody will babysit for you run, run, run. It's a coping mechanism, but it's a good one. IF it works for you, if that's don't your style.
Poor you. I wish I could press Fast Forward for you.