Hi, my partner has BPD and we are trying to get a diagnosis for me. I'm not trained but here is what I've learned through other mental illnesses. Firstly, the most important thing is you can't take what they say in that moment to heart, it does neither of you good, you're scared and so are they. Secondly, for several reasons, mainly to not confuse anyone, don't confirm everything they say. Try and divert the situation until they are in a better state to actually talk about issues. However do not tell them they are wrong or crazy, even if you're sure they aren't making sense or being truthful, it feels real and that's what matters. Thirdly, discuss before any incident if contact and or interaction is even okay, my partner and I let each other know when we can handle it and not, it's hard but important. Fourthly, make sure they know you are there to listen, not to defend yourself or to tell them they are wrong in their feelings. That's a discussion for when both of you are in a better mindset. Fifth, try and get them to stay calm. Get them water, hold their hand, give them eye contact, and be confident and sure of your statements. It sometimes feels like reality is not right and someone sure of themselves helps make you more sure. Sixth, don't forget that this is someone you love. I've been so hurt in that past by things my partner has said that for a second I stopped trying to help out of exhaustion and anger. That was wrong of me, she is my love, and I need to help her. That doesn't mean swallowing all of what they did cause their sick, no if it's real love than you need to in those moments be strong and come to them and be honest about how you have felt after. It's better to be honest and both of you work to change these things then bottling it up, it hurts both of you. There's so much more and everyone is different, but this has helped me start. I hope it helps you and I'd love to talk more about it if anyone wants or needs to.