The only way to fully get over someone is time.
I understand exactly where you're coming from. When my ex and I broke up, I was happy and felt free. But then we started talking again and kept getting back together and breaking up - this happened for 2.5 years and drove both of us crazy. He was always jealous when I tried to move on with another guy. And I would feel guilty and we would get back together. The way how we stopped this was to completely stop seeing each other. I have had to give up some of my friends because he was still seeing them and I just knew I couldn't have anything to do with him even remotely if I was to get over him. It has now been a couple of years since I saw him (last time I saw him, we slept together!) and I am so happy with my current boyfriend, I don't even think about the ex. He has tried to talk to me, but I just ignore it because I know, like you seem to know, that it's over and I no longer want him in my life so what's the point of keeping him around?
I suggest you go to this event. There is the chance you won't run into her if it's a really big event. I still go to events where this is a possibility I will run into my ex but if I did, I would just be civil with him, but not too friendly. This is what I suggest you do. Just go and have fun with your mates. If you do see her, pretend like you didn't and let her come to you if she sees you. Chances are, she may not even approach you. Then you don't have to worry about anything. If she does approach you, be civil with her, but not too friendly and then just tell her you have to go meet up with some other friends and they will be wondering where you are or something along those lines.
If she is with another guy, do not even think about it. You no longer want her and if seeing her with another guy upsets you, it's just your ego talking so you need to shut that voice up and ignore it. Remember that she is your past - other girls are your future.
Have fun at the event and game lots of other girls!
