How does one know? Does it really matter? I think so... I need and want to know. Or can I be both?

I was diagnosed as BPD in 2010 and medicated accordingly... with Effexor. Just last year I went back to see a psychiatrist because I wasn't sure if my med's were even working anymore. He re-diagnosed me as Bipolar II and changed my meds. I did ask him what made me one and not the other and he did not have an answer for me... in fact he said it did not really matter.

I'm not sure how I walked away with an answer like that, I know the two disorders share some of the same symptoms and challenges. From what I have read Bipolar is a brain chemical disorder versus a personality issue due to childhood and the environment at the time. I also know that different medications are prescribed for each... so it has to MATTER.

I'm posting in hopes that someone has some information to share on the topic.

It feels like I could maybe deal better knowing what my disorder really is... who I am . I already suffer from a disjointed sense of self and this does not help.
Is it silly to feel this way?

Thanks for reading