So you are still in the r/s? Are you living together?
You have been researching BPD. A lot of the material here has been developed from the experts in BPD, you may have come across it.
We have feature articles such as this on boundaries:
https://bpdfamily.com/content/setting-boundariesAre you accused of cheating?
A pwBPD at their core feels shame and not only unloved, but they feel that they don't deserve love. This is hard to deal with to say the least
What are your thoughts about going forward on this r/s given that you said that you can't rescue him?