I guess (I really do mean 'guess') that they experience a disorienting sense of disassociation which is comparable, though much more shocking, to the situation of other people who realize, late in life, that their difficulties in relating and connecting to other people are not imaginary.

Most commonly, this is comparable to people who realize that they have been having problems, for years, because they are "on the spectrum" or have Asperger's or autism-adjacent conditions or, perhaps, some ADHD/ADD challenges. I'm sure that there are many people in the HN community who have been surprised to discover this about themselves.

For example, this man, who has been successful in very many ways: https://www.bbc.com/news/world-us-canada-57045770

For people who discover challenges related to gender identity and dysphoria though, the great difference is that they're not only realizing that they have the "wrong" mind, but they even have the "wrong" physical body, too! [In that sentence, my opinion is that the definition of "wrong" is based more on the traditional viewpoints of conventional culture, and not so much on reality].