What a psycho!

So today at work, my manager calls me and the bratty assistant into a quick meeting.
This after the tension between me and snarky escalated once again starting Friday when she lost one of my applications, returned it to me under a binder so I would not see it, ignored me for the rest of the day and walked out without saying goodbye. Saturday was the holiday party and we did not say a word to one another. I was standing in the open doorway at my volunteer post I did not volunteer for and snarky was standing with her arms crossed, back to me, trying her best to pretend I was not there.
For months, everyone around this girl has walked on eggshells, side-stepped her, avoided her, accommodated her, placated her, anything we could do to avoid her nastiness. Saturday night I was exhausted and fed up so after a half hour of being ignored and handing out giftbags to guests while she stood there seething. Until I realized there is no reason I had to be treated that way. So I left.
I swear to God I have not done anything bad to our Assistant. I have tried to befriend her, ask about her interests (which seem usually to be complaints), invite her to do things, introduce her to people, ask for her help, kiss her ass, avoid her completely and try to just keep a good attitude...nothing works.
This morning when it was confronted, she refused to speak about it. She said I had been terrible to her since August, when I was thinking Friday was when the issue began. Yet, she could not come up with a single example of what I had done and told me I needed to look at my behavior if I could not figure out what I'd done. If anyone asks me, I have countless examples of her being rude, immature and unprofessional and could find at least five other people with a moments notice to do the same. I have also spent months trying to figure out what I am doing wrong and thinking about what I can do to change her behavior towards me or at least have it stay consistent. She said I was passive aggressive. Coincidentally, that is what her father's new girlfriend has called Miss Snark, who she is also convinced is out to get her for no reason or result of her actions.
I am exhausted. She was invited to the holiday party for the same reason she was promoted - to mitigate her bitchiness.
I cant control her actions and I am out of options - no wonder I am depressed!
From now on, I refuse to react to her prickly prongs because she is taking away my ability to be myself and enjoy my wonderful life.