I'm not sure whether this or Vampire-chan x Junior-chan takes the cake as saddest non-oneshot Yuri manga I've read. They're pretty close.
This one is really, really realistic. I think that the perspective of the person dying, especially, is conveyed well here. You can tell that underneath everything, Kaori's biggest fear is that she will be irrelevant: dead before 18 and forgotten quickly. In fact, she even says as much in this chapter: "That's good. Something good came out of me existing."
This is the reason she latches onto a girl she sees who intends to commit suicide. Kaori knows she could have a profound impact on Shizuku's life, moreso than most other lives, especially since nobody else noticed how much Shizuku was suffering.
It's... one of those things I tend to avoid thinking about (I think most of us do) because it moves right into the question of "what is the value of our existence? whose lives will we impact?", which is existential depression territory for many people.