Ah, Soup, I'm so glad you brought this to the table today. It's one of those mental cubic puzzles I take out of my mental pocket & ponder now & again.
The best I've come up with is that this "I" has multiple means on every level, not only the physical one. Just as I may sign my name with either left or right hand, so might I approach a person differently. My father, e.g., did not like who I became as I grew up and would deal pleasantly only with the 12-yr old me--so if I wanted something, out came my 12-yr old. I wasn't conscious of this, but looking back I realize that it happened.
Similarly, if I go to Traffic Court, I am a quiet repentive senior. When I go to Immigration with my young friend, I'm an outraged US citizen demanding answers from beauracracy. Am I making sense?
Just as my right hand signs better and my left bears weight better, there are inner "parts" that excel in different situations--& I'm not consciously in charge of which comes out. I think that's where the bipolar issue can mess me up by sending the wrong tool for the job--whereas someone with no MI would always have the right tool at hand.
Just my take on how it works in my life, Soup.

Roadie