Splitting is the abrupt shift between seeing someone as all-good or all-bad — idealized in the morning, devastatingly rejected by night. In borderline patterns it's usually driven by fear of abandonment, not calculation: nuance collapses under emotional flooding. Understanding that doesn't make the whiplash okay for you — both things are true.
Splitting comes from pain and is treatable — DBT in particular helps many people with BPD enormously. Compassion for the why can coexist with limits on the how: you can care about their fear of abandonment and still refuse to be punished by it.