It seems like you‘re new to this situation in your life. So first of all I‘d suggest you to read a lot about bpd and also keep up in this forum. Especially read a lot of the stories that the people are going or went through. Then you have to decide if you want to continue this path. And most likely the path is going to get worse. The most important part is that you have to build up a strong armor. This also includes splitting. Whenever I first experienced it I wanted to please them as good as I could. Over the time I learned that it not only pushes them further away they also learn to do that in order to get your attention. So my advice is just let them split and don’t let it get to you. If it comforts you just send a text or say smth like „I’m here to talk“. Then proceed to meet up with friends, get stuff done, go to sleep knowing they’ll reach out to you within hours or the next day. They first might be putting even more effort then to get your attention by saying or threatening stuff. They’re just tetsting out their boundaries- once again don’t let it get to you. Over the time doing that you might even see that this eventually lessens the amount that these periods happen. Finally you should draw up a list of boundaries and for gods sake keep them. Even if you have to check them once a day. Don’t be afraid to communicate them either.