I recall when I was in a relationship with a BPD how I was always expected to absorb their negative emotions, which is extremely draining when they, by nature, have a negative emotional state most of the time.

So whenever the BPD is upset, angry or anxious, which is pretty much most of the time, it's your job as the carer to feel those same emotions at least to an equal degree, if not to a greater degree.

You could happen to be in a good mood that day, just relaxing, or be doing something you enjoy, but alas, the BPD is upset or angry about something. Now, you're not allowed to continue being in a good mood or doing that which you were enjoying. Now you must share and absorb those negative emotions because if you don't, well, you're just a horrible, heartless person who doesn't care about the BPD. In fact, you're the worst human being to ever exist on the planet and they're going to make sure everyone knows about it.