empath,
I understand
Seriously, "I understand" falls a bit on the invalidating side for exactly the reasons your uBPDh said. If there is one thing one can usually trust a pwBPD - they smell invalidation from afar. They also appreciate validation - good speakers and sales persons typically provide plenty.
"I understand" is a statement about myself and that by itself is not really validating - more the opposite. It is often better
- construct sentences without "I"
- demonstrate understanding and not claiming understanding
In dealings with most people "I understand" will be given a pass and accepted - we however are in relationships with "experts" on validation and invalidation - unfortunately our expert has not yet learned to self validate and avoid invalidating others . Being "validation expert" also does not prevent them from passing expert judgments and demonstrating a judgmental "expert attitude" in the process.