I was in a relationship with someone for over 2 years, and she was showing some signs of mental distress and irregularity even before we started being together. Like she didn't really know her own identity well and she said she felt suicidal at times, and she had some attachment issues, but these weren't that bad (or at least nowhere near as bad as I am/was). But during our relationship I observed more and more behavioural traits of BPD, but they all came about a bit after I started openly behaving in those ways: she cut herself after I did, she started using guilt to manipulate me after I did similar things to her, etc. 

  
We basically just became this toxic mirror to each other that kept draining the other one in a lot of ways. We became codependent and we were constantly arguing (sometimes very aggressively). We broke up with each other so many times that I've lost count while simultaneously both being afraid of abandonement. I felt absolutely exhausted and I felt like she was exhausted too but not expressing it as much as I was. So I ended it and she kind of agreed at first but then kept saying she needs my help and support even after we seperated or else she'll kill herself. And I tried to help for a while but I realised I'm only making things worse because I'm not capable of helping (I mean I can't even help myself properly at the moment), so I had to push her away completely. 

  
And after all of this, I feel so guilty and ashamed. I feel like it's my fault she got mentally weak and picked up my mental issues. I feel like I want to explain everything to her about why I did it this way, but she's just angry at me and we will argue more. I did my best, but I still feel horrible.