Is it permissible to ostracize a paternal aunt's daughter (cousin) to avoid repeating a painful past situation, and due to the psychological harm it causes, knowing that the situation is old and she is not convinced of her wrongdoing?

It is recommended to forgive and pardon your female cousin (daughter of your maternal aunt), and to move past the offensive situation. Forgiveness is a quality of the pious and a cause for Allah's forgiveness. The general rule is that boycotting (hijr) is forbidden after three nights unless greeting ends the forbidden boycott or if connecting leads to harm. The female cousin is not among the kin whose ties (rahim) are obligatory to maintain; rather, maintaining ties with her is only recommended. She must be treated as a non-mahram (stranger); thus, it is not permissible to be alone with her or to look at her adornments.