p.s. I understand the impulse to want to tell the BPD sufferer what they have, and that they need treatment. But everything I've read says that's just futile and counter-productive. Search the web for "Anosognosia and Getting a Borderline into Therapy", and work by Dr. Amador. According to Amador, the only way to get a BPD into treatment is to win their trust and through incredible gentle and humble persuasion, which takes years, if not decades.
So yeah, I think writing a letter to your ex telling her what she is doing wrong ... is likely pointless (and legally problematic, as JRuss pointed out). Dr. Amador says it's not that the mentally ill are in denial. Denial means a person knows the truth yet willfully shoves it aside. Dr. Amador says research has shown that the mentally ill simply do not see their illness. They cannot deny what they do not see.
Me: 50, MLC/WW 45 Young kids Nov 2015: BD1 Apr 2016: BD2 Jan 2017: W filed Feb 2017: D final