It has happened once to me, when I had first started seeing my T. I talked to her about it at the next session, and she promised to try and be more aware of when it seems like I'm dissociating, and try to help bring me back in to the room.

I don't think this has happened to me, but I do think it must be quite common among clients who have difficulties with emotion regulation, as several of the therapies for BPD (schema therapy and sometimes BPD) recommend recording sessions so the client can listen to them later, because they recognise the client's emotional arousal is so all over the place that they won't recall much of what is said. I guess if this problem persists you could talk to T about doing this? I've always resisted it because I HATE hearing my own voice, butthe flip side is that it's quite comforting having T's voice to listen to, and it can be done easily on smart phones now without loads of clunky equipment.

I hope that the pdoc appointment can help you with the anxiety/mania stuff. It sounds like you have a lot on your plate right now