Yeah, nothing says self-respect like an adult holding a grudge from middle school.
What part of forgiveness and acceptance you did not understand?
There is major difference. Forgiveness is act of letting it go and stopping it from influencing your life.
Acceptance is accepting back someone (+ in majority of cases forgiving though not always).
Nobody said that you must accept people back in your life.
Her offence wasn't a light one and author didn't completely redcon the first chapters her offence made a lasting impact.
Just having that person near you will give you flashbacks of those times.
Maturity is to understand that sometimes even if you forgave is much better to let go. Time heals most of the time, but having the offender near you will be a constant reminder of that offence.
And sometimes it's just a stance. People come and go in your life. Sometimes someone who offended you needs to understand that disrespecting someone might lead to losing them for life. (I had friends with whom i decided not to make DEALS (monetary) EVER AGAIN but i communicate with them. On the other hand i also had friends who i forgave and remember our falling out as a humorous one but i will never be friends with them).
As our law system has consequences so does "social" interactions have laws but those laws are just more liquid and personal.
Lastly grudge from middle school, it strongly depends on the magnitude of offense rather than age or grade. I've actually "ACCEPTED" back people who offended me around middle school age only to reaffirm that the source of offense was disrespect which didn't change much, they only became more mature but the root cause of offense was still there. With "maturity" their initial root cause simply became more subtle. I still can talk with them and will greet them but meeting together or drinking together? No. I respect my time and respect myself enough to hang out with people with whom i have mutual understanding.