I had this problem when I was a child. I wanted to be a boy for mayn reasons, and only quite a while after puberty I learned that being a girl has it's merits, too. But I won't deny that maybe some people have so many cells of the other sex (for all the noobs here: this is a scientific fact, in case you underly some fatal outdated biological indoctrination) that it becomes unbearable for them to have the opposite genitals.