This is my first post and I’m not too informed on BPD as a whole, but I really hope to learn
its gonna take some effort, and some learning, but its possible, and doable.
i suspect that your problem is two fold.
1. people with bpd traits have inherent trust issues, and adequacy issues. that she feels that way, in spite of your efforts, is somewhat, par for the course.
2. somewhere deep down is a kernel of truth to what shes trying to get across. the hard part is sifting through the extremes, the over the top expressions of what someone with bpd traits will say, and really, actively listening to what shes saying.
when shes saying this, id encourage you not to argue, not to counter with demonstrations of your love, but to listen; to get at the heart of what is really going on with her, whether it be in the moment, or long term.