First of all, it's difficult to see a past boyfriend or girlfriend with someone new, even if you broke up a little while ago. I feel for you, I really do. I think you should just go to the party with your head held high and enjoy yourself, I know it's a horrible thing to see but you can't let it get in the way of you having fun. Stay with your friends, go and speak to new people and enjoy yourself, don't pay any attention to what he is doing. You two aren't a couple anymore, you might still love him but he has moved on, and has every right to meet other girls. Just go and let him do as he wants, just make sure you don't let it ruin your night. It's nothing something you can avoid forever, the sooner you get this out the way, the sooner it will stop hurting and you will move on.
Secondly, I do think you have every right to know why he broke up with you. It's been a little while now and it might seem strange that you are asking now, but I don't see why you should be kept in the dark. It's fair enough if you want to know what happened, so that you can maybe avoid it in future. To be honest, I would have asked long ago, I don't know how you managed not to ask for two months. If it really is important to you, then ask. It might just seem a little odd to him because you two were meant to have moved on.