Hi Sunflower65, glad you landed here on the boards

Many parents here can relate to your situation, of finally realizing that a young adult child has traits and behaviors of BPD.
There's a sense in which you're right:
Just coming to terms with the fact that I missed the boat and no longer have the ability to get her the help I believe she so desperately needs
if you're thinking of -- "I can no longer make her go to therapy", for example.
There is a silver lining in your situation. While we can't "make" adult children do much of anything,
we ourselves are able to dive in and learn many new tools, skills, and approaches, often unintuitive or that we never knew about before, that can change the dynamic in our families away from tension, reactivity, and conflict, and towards more peaceful relationships.
Knowing that you
can make a difference in your family just through your own relational style -- I hope that brings you some comfort. The first boat (of being able to tell minor children "you're doing XYZ now") may have left, but get ready to get on the second boat, as it were; that one is still available

A good starting place for learning those new tools and skills is our section of articles on
When a teen or adult child has Borderline Personality Disorder. Check those out and let us know what resonates with you, or what you'd like to learn more about.
Here for you;
kells76