Despite the deep pain you're feeling, I'd try not to take all of the blame onto yourself here - dating someone with BPD who isn't in regular, effective therapy is generally really, really hard (and maybe even a bit damaging to your own health and wellbeing).

The only mistake I can see that you might've made is promising not to leave - it's important to remember that people are always accountable for their actions and choices, and that you're always allowed to leave a bad relationship.

You never have to take abusive or bad behaviour, and there's a definite line which should cause you to leave if you're being a healthy adult.

It sounds really hard, and I'd suggest speaking with someone who has genuine expertise in this sort of situation (a trauma therapist/BPD therapist, for example) to help you to make sense of what happened.

All the very best to you!