What do I do when I see ex boyfriend and his stupid friend again?

I went out with a high school friend of mine for 3 years. We were really in love and even talked about marriage and our families loved each other. They were really happy for us. We went to college together and there everything changed. My boyfriend became a total slob, didn't care about the relationship, wouldn't go to class, and failed everything. He must have been depressed but he just pushed everyone he loved away from him so we ended up breaking up sophomore year. It was the most painful thing for me because I still had to see him at school and right way he started dating one of my friends. I felt so betrayed. Even worse, my other friends lived with him in the same quad so they had to push me away too. I was all alone. Then I got into a bunch of fights with my ex's best friend who's a total asshole. This has caused bad vibe between me and my ex, and his best friend and me now deeply dislike each other. A couple of months after I made new friends, found a new amazing boyfriend and am extremely happy while my ex has failed out of school, beat up the girl he was with after me, and is disliked by most people. I don't really want to see him or his best friend but my old friends that we had in common have invited me so warmly to spend 4th of July with them. They warned me that my ex and his best friend are going too. I already said yes, and I would love to see my old friends but I don't know how to act around him or the asshole best friend. There will only be a few other people that I know there and I definitely don't want to look awkward. Any advice? I don't want to start trouble, I just want to have a good time!