It also often means that we have reacted in unhealthy ways to feeling invalidated by the pw BPD.
The latter is particularly true of people experiencing difficult times or a loss and of people who are highly sensitive, insecure, have low self esteem or who are easily intimidated.
So, the first thing to learn in validating others is to be able to identify something to validate in a "sea" of conflict that is both valid and important to the other person. It could be as simple as validating how the other person feels.
Finding a validation target and mirroring it back from the other person's perspective (empathizing) is the crux of effective validating. It could be mirroring back the other person's rationale of how they are seeing things and why they feel the way the way they do.
And all of this may be further complicated by the fact that we are tired, frustrated, fearful, or holding onto resentments.
Even though we know that listening carefully is important in relationships, it can be very difficult to recognize when we aren't succeeding at it.