I can't remember the title of this yaoi that is super cute. This boy thought he was going to marry this girl when he was young. One day, he got to meet his fiance earlier than planned and it turned out to be a guy. The fiance also thought the boy was suppose to be a girl due to the pictures they received when they were kids.
Not sure as it doesn't fit all..
http://www.mangago.me/read-manga/boku_no_ano_ko/
http://www.mangago.me/read-manga/unmei_no_hana/
I can't remember the plot that well but the boy has a fear of dentists because in the past a dentist made the experience really painful. Now, the boy encountered a dentist that makes the treatment painless, which kinda erases the boy's past fear. On another treatment, that dentist's sadist side comes out and makes the treatment very painful for the boy. At that moment, the boy recognized that dentist was the same dentist that gave him the trauma.
http://www.mangago.me/read-manga/haisha_nante_kirai_da/
oh wait no
sounds like the 3rd story in this one (page 50 something i think):
http://www.mangago.me/read-manga/douse_meromero/