I could see it being difficult to distinguish these two when in an episode, which is probably why the DSM says personality disorders should not be diagnosed during any sort of mood episode, unless there is a clear history. I definitely was questioning if I had BPD during a mixed episode because I just felt so reactive and had such a negative view of myself. I think my psychiatrist was as well based on his questioning and the fact that he said he didn't think it was the meds but emotional regulation issues. Except now that I am out of that episode I don't have those problems, and have a much more positive view of myself, circumstances, and am much less emotionally reactive. Although I am sure I can still work on emotional regulation and thought patterns in CBT. I am certainly not perfect, or implying I have no work to do in therapy to manage my moods.