Most Helpful Guy
Act as if she's not there. Show her that your happy, and you don't need her in your life to be "happy". Especially since she wasn't kind when she was leaving you. Act as if your past relationship with her was non exsistant. And act as if it was another woman that did this to you. So if she does approach you, you already know what her character is like and can see through her bullsh*t.
If she does speak to you, says hi or w.e. Give her one word answers, that will not allow her to continue asking more questions, or continue to conversation with you.
If she doesn't approach you and act like a total Bi**h. Then don't even show her that your hurt, don't look her way, make sure you have with someone at the party who is also single, and will keep you company all night. So it doesn't show her that your alone, and have no one. A guy would be good, but a girl would be even better. aka just a girl that is a friend. And if your friend is flirting with you, even better, it will toture her.
Anyways, game plan is:
No contact (unless she approachs you, and you only give her one word answers)
No eye contact
Show your happy
Show she never exsisted
Show your relationship was fake
Chill with a friend that won't leave your side all night guy or girl (theres always people like this at any party)
And have a great time!