Holidays are the toughest time. Especially for the newly separated.
The only thing I can say is you will survive it. Moment by heart-wrenching moment, you will survive.
Your "kids" shouldn't say if you're not going, they're not going. They're not 12 years old for Christ's sake!
I would ignore her birthday. But then I'm kinda hard on walk-aways at times.
Nevertheless...you reap what you sow. And the sooner LBSs start letting WASs reap it, the harder reality hits them and they have a chance then of realizing the ways in which they actually had it pretty damn good.