NTA. no one is under any obligation to attend any wedding other than their own (and even then I guess you're not obligated, but you can expect to break it off if you don't go). likewise, no one can force you to wear something just because it's their day. you gave multiple options, many compromises, and she still won't take it.
I wouldn't attend either, at that point. to her it's just one day, to you it's existing in your body wearing something that's uncomfortable in an anxious situation. to her it's a couple of hours, to you it might feel like much longer. she just wants it to be perfect, which I understand, everybody wants the perfect wedding or whatever. what she doesn't understand is that it can still be perfect even if you're not wearing a dress.