You could say something like, "So my p-doc thinks I have anxiety and depression, but I've read a bit about PTSD and BPD and am wondering if those might fit, too?"

I asked my T once if I could possibly have bipolar II, and we had a good discussion about why she didn't think that I did ("official" diagnoses are generalized anxiety disorder, panic disorder, OCD, and some major depressive episodes). Talking to her about it was helpful though, because it brought up a couple symptoms that I hadn't really mentioned before because I didn't think they were important (they were things I read in a bipolar II checklist).