I’m wondering how common it is for pwBPD to completely misrepresent my helpful actions and kind supportive intentions
Absolutely par for the course. They never see your actions as positive or caring; always negative and 'plotting against them'. It's the ultimate no-win scenario as they'll find some way - in their disordered thinking - to turn your well-meaning acts into threats.
Nothing I ever did for my exBPD was seen as genuine, in her eyes I was always being nice to hide something I'd done or as part of a plan to put her down. Couldn't convince her otherwise.
After they've left - and especially if they've found nobody else to immediately jump into a relationship with - they'll often come back to avoid being alone. Maybe somewhere in their broken minds they do actually know you were nice to them so they come back.. only for the new idealisation phase to fade again and once more you're the villain. In other words, the same treadmill.. and sadly, we're all too eager to get back on it to as we remember the good times we did have and how things might have been.
In the end it simply comes down to when we finally decide we've done our best and no longer want this to be our life, and finally step off the treadmill.