Go. And act like you hardly remember Miss Whatserface. Be cordial but not friendly; warm but not inviting. Say hi then move on.
You're the better woman, you've moved on! Go to what you want to go to. You're a big girl -- don't let her have this control over your life anymore.
Besides, she's probably 70 lbs overweight and has a husband who hates her and a job she can't stand.
