Should I avoid my ex and his new SO at an event with our mutual friends?

I don’t usually ask Fluther for personal advice, but I’m really stuck on this.

My SO of 2.5 years and I broke it off almost four months ago. It was a very messy breakup and we basically haven’t spoken since then.

Every year for several years, the two of us have gone to an anime convention with a group of friends and stayed in a hotel near the convention center. Now, we knew things were going to be a bit awkward with the breakup and everything, but with two hotel rooms we thought things would be okay-ish since we could have some distance, and the group is big enough that we wouldn’t have to interact that much.

Just yesterday I found out that my ex has a new girlfriend. Not only is she several years younger than him and they’ve known each other for about a month, but he’s bringing her to the convention. Needless to say I’m pretty upset about it. I was the one who initiated the breakup, but still… it’s hard to see him with someone else. I spoke to him about it last night (for the first time since the breakup) and he said that he was not going to make any sacrifices to have a good time, even if it meant making me uncomfortable. He says that what he is doing does not compare to how I hurt him with the breakup.

Now, my dilemma is whether or not to stay in the hotel rooms with the group of friends. My sister lives nearby and offered to let me stay at her house, as well as a good friend. Not staying at the hotel means getting back about $100, but it also means less interaction with my friends. One of my girlfriends said that I should grin and bear it, otherwise it shows he’s won. But I don’t want to ruin my experience at the convention and make things awkward for my and my ex’s mutual friends. I was up all night thinking about this and crying off and on; I don’t want to be miserable at an event I’ve looked forward to all year!

Is my ex being insensitive for doing this, or does he have the right to bring his new girlfriend? Should I stay at the hotel and bear seeing my ex with his new SO and risk being miserable, or stay with my sister and leave my friends?

Sorry for the long message, I would appreciate any advice.