But OP has explained that, genuinely, they are. That they supported her through a serious illness, which is a MUCH bigger deal than a bloody NYE party.
I struggle with the tendency, on here, for people to read about one single incident in a relationship/friendship and then go "Oh, they are terrible people, they don't care about you at all, have nothing more to do with them."
You judge people by the totality of their behaviour towards you, not a single incident.
Their behaviour over this was a bit shitty, yes. But at least they were kind of honest - they didn't pretend to be ill or something.....
I have a friend who can be like this. She will bail on me if she gets a better offer, but I KNOW she loves me loads - it's just that she can see me any time she wants, whereas other opportunities may be more limited.
At this point I have said to her, look, we will always be friends, if I ever invite you out and you would rather do something else, please just be honest and say? And I will do the same.
Maybe OP you should try this with your friends, if you are as close to them as you seem to be. Say, guys, I get that you wanted to go to the fancy party - please just tell us so from the get go next time?
That is what I would do. But don't fall out with them over it. It's not worth it. Friends that stand by you and offer support during a serious illness are like gold dust. That's the stuff that really counts - not etiquette over NYE ......