Welcome to BPDFamily, happy to meet you, sorry you're here.
FYI I tend to be more judgmental than others, so while I offer an "alternate take on it", don't assume that everyone or even most here necessarily agree with me - it's just a viewpoint to consider.
"Am I horrible for not wanting to bring her home with me?" - No...and Maybe Yes.
The fact you don't
want to bring her home with you is definitely understandable - even amongst the parents who aren't willing to say they feel that way, most can probably admit the thought has at least crossed their mind if not weighed heavily.
Ultimately what determines your own self is not so much whether you want to, as whether you do things you don't want when they might be the right thing to do. Think of this as a possible opportunity to give her a real-life example of what self-sacrifice can look like, what a focus outside oneself and the present-day can be.
You and her therapist are both concerned about her depression, BPD and subsequent safety, and you're not able to find her suitable mental health supports in her current state where she no longer feels she has any reason to be. In fact you describe it as "impossible" for you, who is higher-functioning than she, to find a suitable safety net for her in this region.
I do think it's great that you've spent a week out of state helping her, but if she's saying she wants to move back with you rather than her father - it would be a relatively major issue in her mind if she perceived you as rejecting her. BPDs are often insanely insecure and their suicidalism often arises from their perception that they are not valued in the way they hope to be. I believe BPDs already have the highest rate of attempted suicide even without accompanying depression and lack of meaningful relationships - so I'd say the risk is much higher for those in your shoes and mine than it is for our friends who may have similarly "failure to launch" children with whom they try "tough love".
If you look around the forum you'll find parents of 13-16 year olds asking whether it's realistic to hope that they'll live in their own place, get a degree, start a family - and the overwhelming number of answers say "
anything is possible, but it's not likely - unfortunately you're going to have to prepare for the fact she may need care through her 20s the way an autistic or schizophrenic child will". It's really rough being a caretaker of a pwBPD - it's not like having a child who is simply a narcissist, OCD or avoidant personality disorder. Those aren't easy - but they don't require the same level of continued care due to a failure to thrive.
The only alternative I would consider, were I in your position, would be gently asking if she'd like help moving to a new locale other than your boring little apartment and helping her get a train ticket and first/last month's rent to go try seeking stimulation in Key West or San Francisco or Fargo; give her a few days to think about possible locations, look them up together and see if they seem fun - but be alert to her feeling rejected and be ready to back down. Ultimately the BPD's likely going to prevent her from finding an identity in those locations as well - but it might buy you some time and make her happy for a few months and help "reset" her problematic behaviors.
In the final analysis, we had children - our children may not have turned out how we hoped, may even be the antithesis of how we tried to raise them - but ultimately don't we feel a debt to them especially when they are still "young"? Again as a disclaimer, I've seen a few comments suggestive that I'm more dismissive of the importance of "taking care of yourself first and foremost" than others think I should be - so it's just my stance, there are others. Assuming you don't have BPD and depression and are in similar straits yourself, then this isn't an issue of "affix your own oxygen mask before assisting others" - it's about triaging to each according to their needs. And pwBPDs have insanely high needs, unfortunately.
Don't forget that the world today is not designed the way it was when you or I were her age - to buy herself a car would require three times as many working-hours as you worked to afford one, college rates are up 1000% and she's inundated by social media and corporations that will attempt to brainwash her into acting like a child without impulse control because that's where their profits lie.
Many 19-year olds today are more comparable to what we considered a 16-year old when we were growing up, or our parents considered a 13-year old to be - ready to start babysitting on their own, maybe flipping burgers a few days a week, but mostly interested in comic books, actresses and discovering new bands that are super-cool. So even without BPD, look at
https://www.businesswire.com/news/home/20130626005441/en/How-Old-is-Too-Old-to-Be-Living-With-Parents-Gen-Z-Says-Age-28-Would-Be-Embarrassing - average teenagers her age (without BPD) say it's okay to live at home until they're 28. One of my dBPDs (
yes there are multiples, happy, happy, joy, joy - it might be why I'm a little less 
and a little more 
than others here - but in my defence, all still alive and not institutionalized) is currently 31 and just re-enrolled in first-year community college transitional courses like "Study Habits to Succeed" and "Learning to Schedule"...and it's not likely she'll end up with a certificate muchless a degree - but it's important that she feels supported and loved and that she's making great strides and improvements (and never finds my account on this forum).
So, with that out of the way, what suggestions can I offer?I'd say her behavior you describe last time she's home is indicative that the depression is overwhelming her - and that's curable whereas BPD is only manageable...so shift some heavy attention onto the depression. Meds are one obvious avenue if you can convince her, otherwise you can try to inject some ambition into her cocooned life. Ask if she'd make a Youtube/TikTok channel if you bought her a webcam, or you promised to appear in every fifth video she made, promise to take her out to her favourite restaurant when she has a video with more than 100 views, etc. Cash bribery doesn't work well with depression because it requires effort to go turn it into something you want and it usually just ends up as 10% more food/alcohol/Disney+ and then disappearing.
Obviously if she's struggling with substance abuse (as many BPDs do), that can heavily impact her depression - so again, consider a "You can move back in with me and don't need to pay rent or anything just so long as you promise to attend the AA/NA/Therapy sessions as scheduled" or something.
Per the BPD, while I'm a believer that medication works more effectively than behavioral therapy conversational techniques (again though, I'm likely in a minority there) - there are two common routes; either "Here's a pill every morning that will make your day less BPD" or "Here's a pill but just take it when you feel an anxiety attack or fit of rage of self-harm episode coming on, you don't need it every morning because you're more healthy than most people!" (though they're actually different meds, but hey, she doesn't need to know that detail - pump up that self-esteem). It's possible she'll consider the one if not the other, or will promise to keep a few alprazolam in her wallet "just in case" and you'll never ask her whether she's taken them, etc. (They do tend to fear judgment or admitting they have a problem, since their identity is so shaky and they think something like BPD is the sum total of who they are).
I wish you and her the best, and hope you're both able to emerge successfully on the other side.
