What should I do when I run into my ex girlfriend with a new boyfriend?

Tomorrow, I will be going to my friend's birthday event and my ex girlfriend of 5 years will be there. She broke up with me over the phone 4 months ago and recently got a new boyfriend a week ago. I feel much better and think I will be able to carry myself better. The first time I saw her was at a restaurant and i felt like crap, but now I think I'm doing much better. I am almost completely over her, but I still have some instances where I think about her so not completely. I haven't talked to her since we broke up and I was wondering how should I act.

Details of event:
1. dinner
2. pre-game at friend's house
3. bar hopping
all of these will be done with a large group including her and her new boyfriend.

We have a lot of mutual friends so I have been kinda preparing for this moment. How should I act?

If you feel comfortable enough to say hello to both of them and go about your night with your other friends, then do that. If you can truly have fun being around your ex and her BF then just hang out and be friendly. You don't have to really talk to them too much or sit near them at dinner etc...

My ex has had to be around me and my BF a few times...at a wedding and a bday party. He kept his distance and just said hello to us once.

If you feel uncomfortable and can't stop looking over at them and feel the urge to get really drunk...then go home and give it some more time. Good luck!