Believing that it‘s possible to change sex, to ‘live AS the opposite sex’, or that you know enough about how the opposite sex thinks and feels to accurately conclude you have the wrong brain for your body excludes you from sanity, IMO.
That said, there is the occasional transgender person who seems to be capable of enough insight to understand that their desire to be the opposite sex doesn’t obligate the rest of us into going along with it.
I have a lot of time for Miranda Yardley*, who is a post op transsexual male and uses those exact terms to describe himself. Perhaps unsurprisingly, his unusually honest position has caused him to be banned from twitter and unsuccessfully prosecuted for transphobic hate crime.
*Miranda was the publisher of now-defunct metal magazine, Terrorizer, so my admiration predates my interest in crazy troons: