Oh how many episodes of Friends talk about this?
I advise you consider what your relationship is like with the ex at the moment, if you're still close to her then you could go, if you're just friendly and don't really speak often then you definitely shouldn't go, it just isn't worth the heartache and she shouldn't have invited you, she probably thought she was being nice but I think it was insensetive, your parents shouldn't go if you don't go, that would be weird.
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'not only has he hacked his intellect away from his feelings, but he has smashed his feelings and his capacity for judgment into smithereens'.