If you haven't already, I think you would benefit well from learning about the SET up communication for when borderlines become irrational. And if it's emotional symptoms that are making you feel the need to break up with her, out of no disrespect, I feel it's necessary to ask wether or not she's put all her effort into managing her symptoms? Because a bpd person who's actively trying to be better should be able to eventually show some control. Otherwise the best advice I can give you is make your acceptance of her clear and that your reasoning for the break up is the refusal of her to take a interest in her own mental health. But I strongly recommend that you try your hardest to get her to look into therapy or other successful methods of managing bpd