To be clear, I am talking about very clearly emotionally abusive texts, not just ranting or insulting or swearing etc. I am talking about when he uses areas of vulnerability confided in confidence and misuses that trust to (attempt to) cause pain or self-doubt.
E.g.
"You just liked the drama of crying 'rape' - I bet you enjoyed it when x's dad did it to you - I think you're just dirty like that"
"It's no wonder your mom and dad didn't like you and preferred your sister - who'd want a slag for a daughter?"
Really heavy-duty emotional abuse.
I can't let it continue.
Those are really mean texts, B-non. I can see why you would find them a betrayal of your trust. They remind me of the meanness of my uBPD mother (long deceased). She was also adept at finding my weak spot and digging into it. And you said in your case this reminds you of your father.
I am also struck, however, at how the texts, though wicked, show that he understands you, and remembers things about you. Many pwBPD (like my uBPDh, for example) do not have the object permanence to use this maladaptive strategy, and/or they are too self-centered to actually remember and process your life story and feelings as you have recounted them. Maybe this is part of the better side of his disposition towards you—he gets you, even though he shows it the wrong way.