Hi ngari
Sorry to hear this about your daughter, I can imagine how difficult this must be for you as her mother to see her having these problems. When was she diagnosed? Did she already get any targeted treatment for her issues before she got sectioned?
I am constantly trying to reasure her that i am with her every step of the way with this battle and that she is not fighting this alone, she says she knows this but i don't think she truley believes me.
Why do you think she does not truly believe you?
... .am constantly either texting her or calling her but i feel she is getting annoyed with this
Do you feel like you are constantly texting or calling her because she wants/needs it it or is it perhaps more because this is something you want/need? Could it perhaps be that you are (also) doing this to reassure yourself that she is alright and knows you care about her?
BPD is a very challenging disorder and I can definitely understand why you are having a hard time with this. That's why I'm glad you are reaching out here for support and advice. I encourage you to take a look at the tools and lessons in the right-hand side margin of this board, specifically the ones about communicating such as listening with empathy, validation and S.E.T. which stands for Support, Empathy and Truth.
It is tough, but now that she's been diagnosed, you at least know what you are dealing with and your daughter can get targeted treatment for her disorder.
Take care and welcome to bpdfamily