Sparing all the details, I'm (M 22) currently really close to my ex girlfriends little sister (Genderfluid, 18 they/them pronouns). I'm not on speaking terms with my ex, but the sister still wants to be really close to me despite everything, but a big problem we have is that we both have BPD. I didn't know it at the time, but it actually explains why I had so many issues in the relationship in the first place. Since the sister wants me in their life, I want to help them grow and get better, and we both actually just started therapy a couple weeks ago. 

Problem is though, the sister gets really triggered when recalling past mistakes I made or arguments I had with my ex. They have every right to be upset, I made some really stupid mistakes, but I don't know how to navigate this problem. I want to help them, but they dig up something we already talked about, or something that I didn't even know was a problem in the first place, and then start a fight with me over it. I don't want to ignore their feelings when they attack me, but I don't want to engage either. Today I tried to disengage by asking for us to take space and I let them know I didn't want to fight and they accused me of trying to avoid apologizing, so I tried to apologize but they kept bringing up more things to apologize for, and I just kept apologizing until I broke down. I want to help them, but I don't know what to do. I tried the SET method, but it only helped in the beginning.