These situations are difficult and I personally try to avoid them.
My only advice is either don't drink or go very slow with it. The last thing you want to do is either go into a rage at her or throw yourself at her like a needy mess. Alcohol will only impair your judgment. Try not to show she has an affect on you and act as neutral as possible. And try to enjoy the event as best you can.
Sadly I have to drink to be able to socialise with lots of people right now. I'll get a friend to keep an eye on me though so I get the right balance.
Right now I want to be in some place were reconciliation is possible in the long term but need to focus on other people.