I believe it's a oneshot and tragic. Starts off where they were already a couple however the uke is already dead. I remember a scene where the seme is about to jump of a cliff because he can't live without the uke and the uke (his ghost) is actually right there hoping he wouldn't but can't stop him because he's just a ghost. Plzz help
This one maybe? http://www.mangago.me/read-manga/ashita_shinu/nmt/c001/
Can't remember if it was yaoi or shonen ai. it was amongst one shots. Seme is a school nurse/teacher and uke is a student who likes him (unrequited love) however the teacher is in love with someone else. Kinda cliche but it's not http://www.mangago.me/read-manga/suki_ni_nattara_dame_desu_ka/

Looking for some younger seme x older uke manga, where the seme has a crush on the uke when he is a kid and chases after the uke after when he grows up. Kinda like viewfinder or Warera no Mizu wa Doko ni Aru or Into Your Heart Through The Door