I just finished reading James Masterson's book "In Search of the Real Self," one of the first to theorize how personality disorders develop (the book came out in 1990, I believe).
It is helpful because it explains in laymen's terms what happens when self-activation, or individuation, or development of the "real" self is inhibited for whatever reason (he says having no "real" self and a strong "false" self is the foundation for personality disordered behavior). I think it's relevant because it sort of explains the family pathology, involving multiple players, when there are "fleas."
According to Masterson, people who are emotionally vulnerable (nature) may be more susceptible to develop personality disorders, especially when there is a primary caregiver who desperately does not want the child to separate (nurture), add to that some kind of separation event (fate) that tests the child's emerging self, which, if the emergence of the real self is not adequately supported, leads to dysfunctional coping mechanisms of the false self taking over. We all know what those coping mechanisms can look like.
Part of the dysfunctional nurture piece is having a codependent parent who won't set limits with a child who has a weak sense of self. It's like the pathology of being paired with a BPD spouse gets played out with the child. Limits are anxiety-producing for the codependent parent, and the child of someone with BPD feels anxiety about having those limits too. Meanwhile, having limits is part of individuation, so the child both wants and resents the limits. So the two support the pathology of BPD in ways that don't look as devastating, while managing to keep the pathology going.
What I see with SO is that his codependent traits undermine D19's attempts to self-individuate and deal with the anxiety she feels about separation, not just physical separation but separation as a concept. SO isn't actively preventing her from developing a healthy self in other ways, but by giving in to (what seems like her chronic) distress, he fans the pathology instilled by her mother. That distress is based on the belief that separation (activation of the self) is somehow linked to devastating abandonment. When in fact, working through the abandonment is about her learning to separate from others in a healthy way so her real self can emerge.
I know SO cannot fix this, it will take a lot of therapy. He certainly isn't helping, though, when he solves problems for D19 that she is capable of handling. For one, I think a 19 year old can figure out that if she buys canned beans, she will need a can opener . She is denied the satisfaction of learning that close others believe she is capable, and she denies herself the satisfaction of learning that she is capable.
Plus, when D19 is needy or clingy, she's enacting the dysfunctional dynamic where being "good" (a definition her mom conditioned in her) means don't separate from mom, cling to mom, need her at the expense of figuring out what makes her different -- and when SO rewards that needy behavior (as it swings toward him), it confirms for D19 that she is good, reinforcing that separation from a parent is somehow bad or wrong. She is now starting to try this with me.
D19 is like your SD, maybe worse -- if her dad sits too far on the couch it triggers abandonment. I need an emoticon with rockets coming out of its head to express how I feel about that level of neediness. That's my stuff, tho. Seeing that dynamic is like setting off a fire alarm that only I can hear. If I point it out, SO acts a little like I'm the one causing the commotion.
To your question about how to communicate the fleas to your SO, I have no good answers. I'm trying different things, using a lot of the skills I learned here on him, like validation, coaching, staying out of the drama triangle, having strong boundaries, putting my needs front and center.
One example: D19 texts SO when she feels lonely, and I try to gently coach him to set the phone down and let D19 work through it. By coaching, I mean saying maybe D19 needs to work through this. Or, "Is this something that can wait? Maybe in 30 minutes she will be able to come up with a solution on her own, and feel good about herself for solving it independently."
I figure SO goes through some kind of separation anxiety with D19 himself.
Any sign of distress from her (which is daily), he is there dictating solutions. Instead of letting her experience the feelings of confronting life situations that produce so much anxiety, coaching her to come up with a solution, and letting her experience the feeling of handling her own problems. I have to be gentle otherwise he can't hear me. Or he nods his head and I can see he is just trying to make it through the moment, without really taking it in.
What seems tricky for us as partners is that we have this clear vantage point where we can see the pathology, being outside the dynamic. I see how we can be emotional leaders. I'm just not clear on how much is too much, and what things to hand off to a therapist.
I keep thinking if I say it the right way, if I explain it just so, if I am patient enough and compassionate enough in saying it, that SO will come to understand the dynamic. I did that with my BPDx about his drinking problem and that got me nowhere.
It may be that we have to deal with the fleas when they directly touch us, and model how that's done? I wish giving speeches was effective