Hi, welcome, Zombilina;
It makes a lot of sense that your SO would "act out" more as you develop healthier boundaries. As you continue to connect and read here, you'll probably run across the phrase "extinction burst":
https://bpdfamily.com/message_board/index.php?topic=85479.0Check out that thread whenever works for you; we'd love to hear if that clicks with your experience.
Does your SO have an official diagnosis of BPD?
Whatever the diagnosis status, this is a place for people who have difficult people in their lives. The traits and behaviors associated with BPD (and other PDs) are extremely challenging whether a person is diagnosed or not. So glad you're here to grow, connect, and share!
Warmly,
kells76