That's wise of you to reach out for some perspective on parenting a young adult child. My H and I have had to do that, too, for the reasons you're describing: when BPD is in a family system, it can be hard to know which behaviors (parent and child!) are normal-range and which ones aren't.
My H's kids are 17 & 19 right now, and while neither of them has BPD, their mom has many BPD traits (and their stepdad has many NPD traits).
I remember a lot about being 19 myself, but my H actually doesn't remember much about being 19. For me,19 seemed to be the age of maximum desire for total freedom/a sense of "nobody can tell me what to do, I can do what I want" with minimum real life experience or wisdom. It's a difficult age to observe as a parent, but a wildly fun age to be

I was coming out of a family background with two married parents who at that time were "generally stable" (my mom had many cPTSD behaviors when we were younger that did impact us a lot). My focus was on having fun with my boyfriend who I met in a 12 week IOP (for an eating disorder), away from parental oversight. I still had some beliefs that I had to ask my parents for permission to do things, but I was starting to realize that I could do what I wanted and nobody could tell me No. I did love my job and was upset that I had to quit it for the IOP, but I was rehired after the IOP, so I did at 19 have a focus on wanting to have a job and wanting to be in college. I wanted to have fun crazy experiences with my BF and friends and my mentality about that was "in the moment" or spontaneous (not quite impulsive, but not deeply thought out). It was all about what sounded fun.
I lived with my grandparents during the IOP but would head back to my parents' house on weekends, so I did choose to use those living situations instead of being determined to live on my own. I don't think I had any conscious sense of appreciation, though -- 19 can be a very self-focused age. I remember my mom disclosing some serious family history to me and my sisters when I was 19, and when she asked us how we were doing with that, I said "I feel like I miss my boyfriend". Of course I look back on that with regret, but at the time, it was a very 19 year old thing to say (what about me, what about my life, what about my relationships, I'm focused on me).
Our 19 year old is holding down a full time job, getting there on time, and is responsive to correction from her manager -- but seems to like to "pick fights" with H and I about stuff that I don't even disagree with her about! She is not very correctable by H or I and does make choices that I totally disagree with. My challenge is to remind myself that she's an adult and the way for us to maintain a good relationship for later, is for me to pick my battles. When she told me one weekend she was going to drive herself to a protest (in the car she is borrowing from us), I did worry that it was a situation that would turn ugly, but I had to just tell her "be safe and call me if you need anything". I have to remember that she knows how to use a phone and knows where I am. She was fine and I think it was good for our relationship that I didn't turn that into a battle of "I don't think that's a good idea".
She also wants to move out of the country to live with her partner overseas. I think that'll be a tough row to hoe but I can remind myself:
it isn't my problem to solve! She has asked for some support (is it OK if I have this piece of furniture) and we have offered some household items, and she has seemed appreciative of that, but the
one time I mentioned "are you concerned about cost of living there" she shot back with "nope!" (even though she later texted her partner "I'm so concerned about cost of living there"). When I can be in the mindset of: my life my problems, her life her problems -- that takes the pressure off of our interactions and decreases conflict between us, which is a relief at least to me.
It would be hard if she was using drugs/smoking pot. I don't know what I'd do in that case. It can be fair for you to set rules for your home because it's your home in your name, not her home in her name. "In this house, there is no pot smoking. Pot can be smoked elsewhere, it won't happen on this property." Not telling her what to do, but sharing information with her that she can use to make decisions.
I even give unsolicited opinions on things like what she's wearing or how much makeup she has on.... and it always end up regretting it. I know for a fact that people I know who are really great parents don't say anything if their !9 yo is walking around looking CRAZY
I feel you; our 19 year old walks around looking crazy, too! She has gotten a lot of tattoos and piercings, and just shaved her head. She also wants to do more permanent and drastic body modifications and I just have to remind myself that each day that she doesn't, she is one day closer to having an adult brain that will be less impulsive and more thoughtful.
Do you think that "appearance" is an area where you could practice the skill of
not being invalidating? Drugs, relationships, work, therapy, might all be harder areas, but maybe the area of appearance could be one where you tell yourself: "I may find myself saying my opinion about other things, but this topic is where I will practice decreasing invalidation. And, a more validating home environment will be a relief to us both!"
Not having to "fix everything", and just focusing on one area, could be a way for you to feel like change is manageable

...
I do ask SD19 "curiosity" questions: "how do you decide what tattoo to get next?" "how are your new piercings feeling?" If I don't feel up for discussing the pins on her jacket or the slogans on her shirt, I don't. I am a limited person and I often don't have the energy to "get into it" with her about her differing opinions. She can be defensive right off the bat so I just have to gauge what I'm up for, and honestly, most of the time, I don't engage.
19 can be a time of a ton of energy and a sense of discovering the wider world and its problems, so many 19 year olds can come across as know-it-alls: "I'm the only one who sees what the real problem is, and unlike you adults, I'm actually going to do something about it!" Not sure if your D19 has that tendency, but she may have a sense of "I know how I want my life to go and you are just stodgy and don't really understand". This can be a normal range developmental response that pushes a young adult child to differentiate and seek independence.
When BPD is in the mix, that may change, but underlying desires to "push back" on parents (while still receiving and accepting parental support) is normal.
We have some friends with a 23 year old and the mom said that 19 was totally crazy but 21-23 you can start to see a real settling down. Their daughter was posting scandalous pictures on Instagram at 19 and the mom was sharing with us how she had to remind herself "my daughter knows what we believe, and knows how to get in touch with me, and knows how to ask for my opinion -- I don't have to pursue her, remind her, or correct her, I just have to ride it out and be someone she can come back to". And, she said there has been a night and day difference between 19 and early 20s. Her D has settled down and isn't behaving inappropriately any more.
It really helped my H and me to hear another parent say "yeah, 19 is very difficult and very impulsive, and it can get much better".
So, to your story --
it is natural to worry!
it is natural for a 19 year old to both push back on parents a lot, and to, without obvious gratitude, still accept and receive parental support
it is natural to feel impulses to say things to try to help your child, and it is natural for the child to react to those things
it is natural to wonder if we are parenting OK (I have a lot of past actions that I regret)
it is natural for 19 year olds to be insufferable know-it-alls
it is natural for 19 year olds to "spread their wings" and try a lot of behaviors
it is natural for 19 year olds to be very self centered
it is natural for this to be a hard time for parents

many 19 year olds, once they are in their early to mid 20s, do settle down and reconnect with parents
There are probably many other perspectives from parents on these boards -- there can be a wide range of 19 year old behavior and a wide range of parent approaches.
What's really important about your situation is that you're looking for ways to "know better and do better". That's all any of us can do.
Hope there was some helpful food for thought in there;
kells76