Here is my personal opinion. If she is being this selfish about YOUR party weekend, you are better off w/o her. You don't want to force someone to have fun with you. It's just not worth the effort and she is not worth trying to convince. Just brush it off and have fun on your own and NEVER let her know you are disappointed. She will feel bad and left out all on her own when you guys are doing your own thing and have stories to tell and fun pictures to show. You basically gave her a little bit of rope and she has hung herself with it. I understand her wanting to spend time with her significant other but they would not be there if it weren't for you. The weekend plans should revolve around you and no one else. She could of easily arrived a day earlier or left a day later to spend alone time with him.