Authenticator

I have never been person who could just ignore someone else who was bothering me.
I feel sick to my stomach when I have a conflict with someone that has not been confronted.
My least favorite kind of fight is a passive aggressive one.
Our snarly assistant, nay, newly promoted coworker in my office is terrible. 99% of everyone in thinks so - except for our boss.
On Friday she lost an application I needed, insisted she never had it and never apologized or acknowledged it when it appeared, and then returned it to me under a binder to look like I misplaced it. She then tried to use carpooling with me as the reason she could not volunteer at an event. When I asked her about it, she said, "Nevermind". It remained unresolved and we did not speak a single word to one another for the rest of the day or at the event the next day.
I had dreams about confronting her all night.
Why is it so hard for me to ignore her and let her nasty behavior go. I see that she is being nasty and it tortures me taht everything is below the surface in the passive aggressive, miserable lair where she chooses to slink along. I know she is unhappy so I try to just feel sorry for herself but at some point I want some justice. I work too hard to deal with this crap and I have waisted too many hours worrying about what she is doing.