I feel like you might've read most of them so I'll just recommend two. First being 1866( short, cry) and the second being My lover, my teacher( it's fairly average, not considered long but not short either, sob).
Enjoy!
"Meaning of his gaze"
It's not really a sad story but my heart ached a lot while reading that.
I don't have a specific read, but I did look it up for you. https://www.mangago.zone/tag/Unrequited/
https://www.mangago.zone/read-manga/the_third_ending
https://www.mangago.zone/read-manga/limited_run/
https://www.mangago.zone/read-manga/bbanana_scandal/
if you need the link for the third story dm me...
READ SANGYANG:THE WANDERER!!!!!!!!!!!
https://www.mangago.zone/read-manga/sangyang_the_wanderer/
Hii please recommend me BL unrequited love that hurts your heart :3
Please be completed and please comment if it'sa short manga or a long manhwa
Thank you!