I could be just plain old "human nature." There are plenty of people who don't have any willingness to admit to wrong-doing. A person doesn't have to have any psychological disorder to be that way. It's almost a natural inclination that all humans have, but some grow out of, if they get the right moral training while growing up.

There is nothing wrong with resenting someone who has done wrong to you. That may be a healthy mindset that keeps you motivated from getting involved with her again.

I think you will find it possible to let go of resentment when you get to where you no longer see her as a threat to your peace of mind. That might not happen until one of you gets employed somewhere else. It can happen sooner, if you figure out how to keep those boundaries up, so you don't keep getting re-victimized by her.