Addy, I thought of something else when I went away just now.

It's different I think if someone is actually setting a place at a table anticipating our arrival for dinner or something, but when it's an open house or a very casual gathering, I sometimes just say thank you very much, that sounds great and I will come if I can. That leaves the situation open (and without excuses) and then I can decide at the last minute if it's something I am able to do, or really want to do or not. By the time everyone gets chatting and enjoying themselves I've found that my non-arrival at those types of casual events isn't often noticed. lol

Sometimes I do go for a little while. It depends, but at least there is no pressure.

This situation is ideal really because it doesn't cause anyone any grief or anxiety and heck, that's what it's all about. We can do without having to feel badly about not arriving somewhere when we don't really want to be there.