Then we went into new territory; said I really never liked her that much, that my other relationship (13 years) was better, and this one wasn't much of anything. All of this was after she said I was bad at relationships.
I know how hard it is to not respond when your buttons are pushed ... .my ex was a pro at pushing my buttons. But I always felt guilty (and still do) for letting her push me over the edge. It is not who I am nor do I want to be (goaded or not).
Given the circumstances of shared custody this was probably not something that should have been said, angry or not. I think you know this. I believe anyone, disordered or not, would be extremely hurt being devalued like that. For a borderline it is probably way worse. What done is done but you will probably need to find someway to repair the damage done because there will potentially be some backlash as a result.
My advice, as hard as it may be, is to not let her bring you down to her level. You have to interact with her because of the kids and you need to find a way to do it without all the conflict. Nothing good will come from continually letting her drag you into these arguments and confrontations. If you can't remove yourself from the situation or diffuse the situation with validation, then IMO silence is better than lashing out and throwing fuel on the fire.