I think that it is a little ridiculous that she is pressuring you in to going. Could you tell her that you already have plans? Or if you truly feel that you must go, could you only go for a little while and then leave early? That way you have still gone and you friend can't be mad at you for not showing up. I really empathize with you. However, in the end it will have to be your decision. Your "friend" cannot force you to go (and it stresses me out right now to even think about how she spoke to you "You had better make the effort and turn up." etc. If someone had spoken to me like that, I would have told them a) that I did not appreciate being spoke to that way and b) that I would attend if I did not have other plans and c) if I was feeling up to it.) Bottom line, it's up to you whether or not you want to go. If she does not understand, then explain to her that going to parties is too much.
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--Nyx-- What an astonishing thing a book is. Across the millennia, an author is speaking clearly and silently inside your head, directly to you... Carl Sagan