Things didn't end super well with this person and I'm not proud of myself for how I handled things, I somewhat "ghosted" her when this happened and having now experienced this to its full extent at the hands of my BPD ex, I'm feeling some shame and regret about how things ended with this other person. I haven't communicated with this person in any manner for almost a year and a half but I'm wondering if writing a short email to her to express my gratitude for the good times we had & apologize for how things ended would be a good idea?
I say it depends what your motives are. And is it really shame you're feeling, or guilt? We feel guilt when we think we did something wrong, we feel shame when we think we are wrong. Anyway, if it's to apologize for whatever happened and genuinely wish her well, that could work, and consider how she would receive it, if it would benefit her or hurt her. And you might consider hand writing a letter instead of an email, more personal and maybe appropriate for someone you had an intimate relationship with for 6 years?
I've communicated with previous girlfriends after time has passed and the emotions have been processed, sometimes it has worked out well, other times not, and sometimes it became clear all over again why we aren't together; again, it goes back to motives and where we are emotionally around that relationship.
Another benefit, having graduated borderline school, as you mention, is we look at the world, people and relationships differently, having grown and gotten wiser, and it can be interesting to revisit old relationships with different ears and eyes, but again, not at the expense of someone we hurt and may hurt more. Your attempt could be interpreted as an attempt to reconcile, which you mention it isn't, or an attempt to clean up the past and tie up some loose ends, apologize maybe, or maybe a pure attempt to reconnect to wish her well; it's good to consider how it will be interpreted too.
