Thanks, everyone. I think I'm going to have to come to accept that the doubts are part of the cycle.

I decided to talk to my therapist about the doubts and my suspicions that it could be something else, like BPD. He told me he sometimes doubts his patients diagnoses and suspects their psychiatrist may have overlooked something, but he is 100% positive I am not borderline and that I have Bipolar I disorder with comorbid anxiety, most likely GAD. I trust this man more than any therapist I've had, so that helps. He also told me it's very common for bipolar patients to doubt their diagnosis from time to time and that it's not uncommon to suffer from severe anxiety with it.