Sounds like you want to go, but you're angry that your ex and his new g/f will be there. Hard call, but you could decide to go and then leave if it's too uncomfortable.

It sounds like the majority of the people who will be there are ones you'd really like to see and to be with, so why not go with that in mind and let yourself have fun!

You can make plans for if it doesn't work out and you don't want to stay. Such as having a nice take-out like Mouse suggested.