That has been perhaps the biggest struggle for us @Beermoney .... the fact that you can't try to help them to see what is happening through their smoke cloud without risking the relationship .... and that is, in fact, one of your choices .....
Always remember that you do have the choice to leave .... I had the choice to leave, and I can still leave even now .... but that would have had consequences of its own, and that is part of my reality.
Usually choices are not about the choice itself. Every choice has consequences and it is the consequences you need to weigh up and work out what you can live with. For instance, telling the person that you love them, but you won't live with them the way things are can sometimes cause them to seek advice or support, and in doing that it begins to show up the true nature of the problem .... but they might turn around and leave you instead, or not believe that you love them and tell you to go ....
It ain't easy ....
Advice I was given by e.d. support services was to learn as much about my hubby's eating disorder as I can (without jumping at every shadow) because it helps you to see past the illness to the person trapped within it. You can begin to see the purpose or path the illness is taking, and the behaviours and reactions start to make sense within the context I the illness, instead of being just random events without rhyme or reason.
There is also a behaviour model that can be used to understand the changes that have occurred, and where they are heading ..... learning about this model really gave me some hope when I didn't know how to hold onto any ..... because I have heard of eating disorders lasting a lifetime. This Stages of Change model helped me to see that the first stage of change - marked by denial and hostility - doesn't last forever, which means that the person does eventually wake up to the fact that they have a problem. After that, they need to become active in trying to overcome it.
And that can result in a lifetime of struggle.
If you choose to stay with her ..... get educated about what you are dealing with .... go into it from here with your eyes open. This link takes you to the Eating Disorders Victoria website, so the Stages of Change model is spelled out for eating disorders, but the same stages apply to disorders in general. It sounds like your lovely lady is in the first stage of denial and hostility.
https://www.eatingdisorders.org.au/eating-disorders/what-is-an-eating-disorder/stages-of-an-eating-disorder
This book also helped me, even though my hubby doesn't have BPD. I go to see a psychologist every few months for emotional support to live with our situation. He said that the general principles offered in the book, about how to deal with dysfunctional behaviours, will help me anyway, and they have.

Hope this helps. Here if you want to chat.