Why do you KEEP engaging with toxic others?? It's pure Egoism.
Not only do you feel a strong need to have the last word, your interplay with 'em gives you a great deal of satisfaction. It codifies your disdain and loathing for that other, because they continue proving you right about them not being worthy of your time or attention~ AND, it gives you something to bitch about.
If you keep needing to confirm for yourself you're right, there are deep insecurities related to BPD traits, that are driving this reflexive need in you. Do you think you'll EVER feel strong enough to surrender it?
Truly empowered people don't crave or need this indirect validation. They've come to accept and integrate that another can't/won't provide any nourishing, sane or useful contributions to their life, and while they may struggle a bit with cutting off contact because "hope springs eternal," they DO it, in order to quit deterring their progress toward genuine wellness.
So, WHY do you keep that door open for a toxic other?? Because there's an EGO Payoff in it! You get to tell yourself you are better adjusted, healthier, brighter and more sane than they BY CONTRAST~ but your need to keep this cycle in place, merely highlights your long-denied insecurities and under-developed sense of worth and Self.