I am rejecting all of Once's four points and replacing them with a set of my own. These are open for discussion and I would encourage you to submit your own as well.
Ad, to be very clear, these are not my points, my thoughts, or my theories. the views expressed are not those of once removed

1. we mate with our emotional equals.
this is the basis of bowens family systems theory.
bowen was a physician during world war 2. the very short version is that he wondered why some soldiers came back from war worse off than others. his research revealed that it boiled down to an individuals level of self differentiation - self differentiation is synonymous with emotional maturity.
similarly, take a loving family that, for example has three daughters. two navigate life okay, or great, one develops bpd. why?
similarly, take four people that date someone with bpd. one gets out quickly. one struggles, but after processing their grief, bounces back. one struggles, for years, during the relationship, and afterwards. one never gets over it and a deep wound develops. why?
this is one, when ive seen it shared, or shared it myself, that gets a lot of pushback. i dont know if thats denial, or misunderstanding, but i honestly think a lot of it is the latter.
it was a hard concept for me to grasp. my ex and i did not manifest at all in the same way. she was prone to emotional outbursts, lashing out at people, higher than average jealousy, things im not. so how were we the same emotionally?
navigating life on a similar emotional plane is not the same thing as acting alike. you can have two very needy people, for example, with very different kinds of needs.
additionally, our level of emotional maturity is not something that is fixed or static or permanent. we can grow.
more here:
https://bpdfamily.com/message_board/index.php?topic=275714.msg12612781#msg126127812. 50% of romantic partners of a person with bpd have a personality disorder themselves
its challenging to find a good source on this because most of my searches yield results that are about people with BPD.
i dug this up, but you have to pay to read the full article:
Romantic Relationships of People with Borderline Personalityhttps://www.karger.com › Article › Fulltext
by S Navarro-Gómez · 2017 · Cited by 9 — An 18-month longitudinal study found that more than 50% of the partners of people with BPD had a personality disorder at the end of the follow- ...
if youd like more, i can find it. its buried here on bpdfamily somewhere.
importantly, though, this isnt to suggest that 50% of members here have a personality disorder, any more than it is to suggest all of our exes have one.
most of our partners had traits of BPD, or another mental illness. most of us have traits of a PD, or another mental illness. 30% of the population, at any given time, struggles with mental illness (which may include short term mental illness). that number is much higher here. around 80% of the members here arrive clinically depressed, and experiencing cognitive distortions.
but yes, there are members here, on this very board, with personality disordered traits, or a full blown personality disorder. i think its something worth exploring about ourselves (i may not be "a narcissist", but i have learned a lot about my blind spots by discovering my own narcissism), and something to consider when we help others reach for detachment.
3. most of us were not with someone that would qualify for a diagnosis of BPD
this one also gets a lot of pushback, understandably. we are all here because we were, or are, heartbroken over a very difficult person that we loved, and suffered over, and an explanation for that suffering means a great deal.
if your ex were diagnosable, they would most likely already be diagnosed.
there are lots of people with borderline personality. and then there is somewhere between 1-5% of the population that has borderline personality disorder.
borderline is a personality style. a set of traits, characteristics, and common behaviors.
those things become a personality disorder when those traits are causing severe distress and impairment on ones life. an eating disorder, a suicide attempt, seriously reckless behavior that lands you in psychiatric help that, along the way, diagnoses you and attempts to treat you.
it has nothing to do with the difficulty you cause in the lives of others, and everything to do with the difficulty you are personally experiencing in your life as a result of your personality traits. there are millions of difficult people that screw over everyone they ever associate with, but dont have a personality disorder.
its a little bit of a semantic difference. you can have subclinical bpd traits and be a more difficult, more destructive person in the lives of others, than someone that would, for a variety of reasons.
have you ever taken some time to read the son/daughter board? these are loving parents posting about a son or daughter with a far higher degree of dysfunction than most of us experienced.
ILMBPDC and I have had a discussion of just what it is that our BPDex's did to us with their mirroring, etc...It triggers a "self-love" that is inside of us where we are essentially trying to love ourselves via our BPDex. It's so complicated I can't really put words to it, but it relates to the emptiness of our ex's and pulling us in and mirroring our own wants and drives back at us. What I'm talking about here doesn't relate to any dictionary definition of narcissism or any other term that I've yet seen developed. Yet, it was something I experienced.
mirroring is a normal human behavior that facilitates bonding. if i laugh at your joke, if i nod as you explain something, im mirroring you. if i validate your point of view, if we laugh at the same family guy joke, we are mirroring each other. in fact, according to freudian theory, personality disorders develop/we fail to develop a sense of self when our mothers fail to mirror us in infancy.
this is one of the most commonly misunderstood bits of terminology here.
it is common - extremely common - to hit it off with someone, to bond with someone, for whom you seem to have everything in common at first. you love everything they love, and they love everything you love. to be attracted to their quirks, to idealize what you will eventually find incredibly annoying. this is the honeymoon phase of a relationship when we all have rose colored glasses.
people with bpd traits just mirror other people at an above average level because theyre insecure, lack a sense of independence, and are desperate for approval.
people that date people with bpd traits (in general) have an above average level of need to be mirrored. the description of the lonely child is just that. they struggle when that mirror is not consistently held up, and struggle to get it back. we arent victims of a well thought out tactic. we overinvested in a fantasy.
having said that, i have read this notion that "we fell in love with ourselves" many times. for starters, i would ask if thats realistic. was there really nothing you loved about your partner? if so, what were you doing?
and if what you were doing was falling in love with yourself, what healthy person gets into a relationship to fall in love with themselves? doesnt that scream "narcissism"?
so, there may be a kernel of truth to the notion for some of us. there was a heavy investment and fantasy in how someone made us feel, and part of our struggle has less to do with the loss of them, and more to do with the loss of that, and what it subsequently exposes. but im always confused when i see it expressed as something to pat ourselves on the back for.
1) The pwBPD is an Emotional Child who can't handle their own emotions let alone understand yours. This makes them a volatile and unstable person, incapable of higher emotional states such as empathy or true love.
2) They seek out people that have something they lack. In my case it was my Self-Concept and self-confidence. She thought (as many others do) she could get it by cutting me down and taking it from me. That's not how it works.
3) The Monster (Mr Hyde, etc...) wants to destroy you. Once that emerges it cannot be negotiated with or otherwise cajoled. It wants to annihilate whoever is in it's path. You being their romantic partner are likely the prime target.
4) They have some control, enough to change their behaviors or seek help. Most chose not to.
Ad, respectfully, these are feelings about your experience (there is not a great deal that is factual to them), and i would challenge you (all of us), at this stage, to broaden them if you want to heal and learn the lessons you want to take into future relationships.
if people with bpd seek something in us that they lack (seriously, who doesnt?), what is it that we are seeking? what exactly do we get from trying to make another person complete?
I've often told my therapist in therapy that I attracted my BPD through wearing a mask because I don't think my true self is worthy of love.
youre onto something, grumpydonut. keep digging.
the most ironic thing about my recovery was that when i realized how similar my ex and i really were (not necessarily specifically, but operating on the same plane), when i learned more about who i was and what i was blind to, how i saw myself vs how others saw me, put together all of my other previous failed relationships, it disabused me of the notion that i was a victim of anyone or anything, and that, in turn, helped me to heal, and to free myself from continuing to make the same mistakes. mind you, that took a few years, and some more continuing to make those same mistakes, but once i saw it, i couldnt unsee it.