Fear, obligation and guilt means we become accustomed to being their saviour however we are not at all helping them. Your relationship with her sounds enmeshed/co-dependent – you really do need to step back for your health and for her.
BPDsis needs to learn how to self soothe rather than rely on you to do it for her. If you continue to save you are setting up this relationship for future rounds of saving. She will threaten, she will scream, she will go silent on you. The only thing you can do is validate her emotions, not her behaviour and work on your guilt for not being there 24/7.
I used to see a therapist the previous year briefly when she was at her worst last year. I stopped seeing the therapist because my own schedule got too busy (and my sister "got better" after the winter because she was back with the BF again).
You matter and there are reasons why you are placing yourself in the position of your sisters keeper.
I know everyone's logical response is to stop taking responsibility for my sister's choices and to take care of myself -- our mother even says it now. I struggle a lot with this. I feel like if I allow myself to let go even just a little, that I could be letting go over her completely.
Any ideas why you struggle with this?
Is about control and the fear of loss of control?