Waka Hirako’s drama, My Broken Mariko. It’s not a oneshot, but it’s not too long:
https://www.mangago.me/read-manga/my_broken_mariko/
This one’s not as good, and it’s more twisted but it’s a oneshot
https://www.mangago.me/read-manga/die/ reply
“Kanojo to Kanojo no Neko” is has been awhile since ive read it but i did skim it a bit and i think it might work for what your asking
this very much isnt a oneshot but it was the first thing i thought of when given the themes of depression/suicide and that is “Goodnight Punpun” its much longer than a oneshot with 147 chapters but it is an...... reply
