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Below are my recs along with my interpretation of the kind of sadness they portray ^^
* Philosophia (reason that cant be comprehended): https://www.mangago.zone/read-manga/philosophia/
* Tsuki no Uragawa (will we only ever think abt it when its already over?): https://www.mangago.zone/read-manga/tsuki_no_uragawa/
* You Are My Angela (the dark side of love): https://www.mangago.zone/read-manga/you_are_my_angela/
* Jewel of You (unrequited love): https://www.mangago.zone/read-manga/jewel_of_you/
* .925 (very short oneshot, I'll let you find out :): https://www.mangago.zone/read-manga/925/
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I deliberately skipped this one but now had the urge to include it here too so there we go: https://www.mangago.zone/read-manga/seasons_in_bloom/
Hope you find sth you like ^^
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https://www.mangago.zone/read-manga/run_away_with_me_girl/
https://www.mangago.zone/read-manga/only_need_to_love_once/
https://www.mangago.zone/read-manga/vengeance/
https://www.mangago.zone/read-manga/so_do_you_wanna_go_out_or/
https://www.mangago.zone/read-manga/outro/
The ones' Shan mentioned are good as well.
sad wlw recommendations please!