Hi! I don't know this specific manga as well but two persons made a list about long shounen-ai+yaoi mangas and some on the lists are from the 90's so hopefully you'll find that one manga you're searching for:
http://www.mangago.zone/home/mangalist/1759/
http://www.mangago.zone/home/mangalist/29445/
Good luck!(๑•ㅂ•)و✧
Okay, I am embarrassed that I forgot the name of this story, when it was such a big deal and went on for so long (last chapter printed in 2000.) And it's only about 20-odd chapters, not 120 ... sorry (when I read it last time, it felt like 120 chapters.)
It's Koide Meiko's Yatteraneeze: http://www.mangago.zone/read-manga/yatteraneeze/
Still classic, old-school yaoi, mind games and all.
I'm looking for a multi-volume yaoi, or possible shounen ai, series that came out in the 1990s (not Kusatta Kyoushi No Houteishiki, not Eroica ...). The yaoi didn't arrive until about 90 - 100 chapters into the story. It mimicked KKnoH a lot in that it, also, was about a schoolboy who develops a crush on an older gay man and is rejected outright, many times over, for various reasons, but mainly to do with the fact that he is too young. He persists, nonetheless and eventually grinds his beloved down. The art style was a little stiff, like Banana Fish, but that was normal for those times. Does this ring a bell for anyone?