“after she was horrifically nasty to me over a misunderstanding.”
“a bad “neighbor”
Pirkei Avos did not mean that a single nasty comment warrants one to be categorized a “bad neighbor.” Check the meforshim there to see what it really means.
Now that you understand that she had been under the stress of a difficult marriage, perhaps you can be melamad zchus that she responded out of character (not a justification but an understanding). None of us is perfect and we all make mistakes.
I wonder if you brought up the incident so that she has a chance of doing tshuva & apologizing. Why not be up front and tell her that this has bothered you ever since it happened.
Honesty many times is the best policy.