One suggestion would be not to go to an end of the world party for two. It just doesn't sound fun. Is there kool-aid involved??? A process server??? Maybe it sounded better in the invitation.
Seriously, though, it's okay to be open to some invitations, but not all of them. It was a good thing that you had a conflict and wouldn't change your plans to suit her. It's not just about impressing upon her that you have other things to do. It's also about valuing yourself.
As for how to keep cool about things, it's as easy or as difficult as detaching. It may be that since the two of you have been so close, she's going to try to pull you back in every time she feels you moving away. She may be having as hard a time with this as you are, and it may be up to you to be the tough one and make space for both of you.
Also, when you are with her, try to keep in mind that you want her to feel comfortable around you. Like a safe harbor. Don't talk about the R. Like you said, just be.
