I think it's because she has a personality disorder, to be completely honest. She follows the patterns for NPD or BPD to the letter...more so BPD after trying to learn about it for over a year now. This letter was a while before she left.
What I know is that somewhere around the time of that letter, she had also been having feelings for one of her ex bf's (they worked together), also one of my coworkers (that she met about 2 weeks before leaving me). I had suspected for a while that she was talking to other people. I can't confirm that there was a PA prior to her moving out, but she wasted no time once she did move out, and there were plenty of sketchy things that she would do for the past couple years before moving out that made me question whether or not something was going on. She would spend time with her friends (including guys) drinking after work, go to parties (found a Whisper post of hers saying 'Going to a party and my ex will be there, IDK if I'm nervous or excited). So I'm pretty positive that at some point along the way or shortly before she left there was a PA. She admitted there was an EA going on...probably more than one.
As bad as I wanted to save all of this, at this point I don't think it's worth it. If she's BPD, then she'll need years of work and therapy to get better to the point where she won't sabotage relationships. And I don't think she's going to do that, to be honest.
BPDs are inherently unhappy with themselves. They believe things like "I fear that he will abandon me, so I'll leave him...that way he's not leaving me." It's just...crazy.
I got a message from W a couple weeks ago stating that she has been seeing a doctor for anxiety and panic attacks. I know that in the past she has been suspected by a therapist of having a personality disorder. So...I don't think I'm really that far off on my estimate.
