Thank you for writing this essay, Marie.
It is a sobering read for all who are idealistic enough to project the possibility of transformation onto people who communicate through their behaviour that they are incapable of it and have no wish to reconcile or show even the slightest bit of regret about something unwell they participated in a long time ago.
Your mentor is very wise.
You have made the correct decision to unfriend your former bully.
There are a lot of people in this world who will be in your corner and contribute positively to your life — it is good to gravitate towards those people.. and close the doors to the past.