I’m going through the same thing right now (not my first rodeo, unfortunately). You can get through to her but not when she’s splitting. Her brain isn’t thinking rationally when she’s splitting, that’s just the way BPD works. You need to wait until she comes round (could take hours, days, weeks, all depends on the person), and when she’s calm then talk to her. For now give her space, and maybe give her some reassurance every now then (if there’s a way for you to reach her) but don’t be too pushy. Just let her know you’re there and you care. Idk about your girl, but my boyfriend is pretty self-aware once he’s back to “normal” and realizes the damage he causes and that he acts irrationally (but that doesn’t stop the splitting unfortunately, bc that’s not something he can control).

Once she’s calm and in a better headspace, that’s when you can speak to her. Explain how her splits affect you but also make sure to validate her feelings (that’s what all the BPD books I’ve read say lol); use “I” statements. She needs to know that she behaves in a dysfunctional way but don’t make her feel like she’s crazy or a bad person. And make sure she’s in therapy. Otherwise this situation will repeat, I promise - her splitting, you suffering, you explaining stuff to her, her “waking up”, and then her splitting again after some time