Yes, these relationships are very difficult. Learning some tools to navigate them can help, but the bottom line is that living with people with BPD will always be challenging and if you’re looking for smooth sailing, well...
In the beginning these types of relationships seem magical and there’s a reason the term “honeymoon phase” exists. Sadly that time doesn’t last. You say the sweet sincere person you know is gone and replaced with someone you don’t even know.
He is
both of those people and the more time you are together, the more you’ll get to know that stranger.
Here’s an article that might be helpful:
https://bpdfamily.com/content/how-borderline-relationship-evolves