Okay I'll give you recommendations but I can't remember all the manhwa names so I'll give links for those that I know -
https://www.mangago.me/read-manga/10_years_where_i_loved_you_the_most/ (This one definately has that feeling where the MC and ML are already in a relationship, but the ML spends more time with someone else so the MC feels like second option. The ML is an asshole so they don't end up together (thankfully). Make you feel sad.)
https://www.mangago.me/read-manga/cosmic_boy/ (Feels like unrequired at the start, later becomes required. Not a love triangle. Makes you feel sad.)
https://www.mangago.me/read-manga/keep_holding_on/ (can't remember the plot, but I think this also has some sort of unrequited love)
https://www.mangago.me/read-manga/until_i_meet_my_husband/ (Feels unrequired for the two guys MC liked at first but, later becomes required when the ML arrives in mc's life. Make you feel sad and happy.)
https://www.mangago.me/read-manga/protect_your_purity/ (ML and MC like the same guy but, ML and MC starts to fall for each other and thinks that it's unrequired.)
https://www.mangago.me/read-manga/bad_contact/ (MC liked someone that doesn't like MC for many years. ML tells MC that he also likes the one that MC loves. MC is heartbroken; later MC and ML gets together)
https://www.mangago.me/read-manga/reunion/ (Not a love triangle. It's a really good manhwa so I recommended it. It does feel unrequired at some point because the MC thinks they can't stay together even though they love each other so much. Makes you feel sad and happy.)
https://www.mangago.me/read-manga/green_apple_paradise/ (Can't remember the plot but I loved it. I think this has unrequired love too because I put that tag when I saved it.)
https://www.mangago.me/read-manga/incidentally_living_together/ (MC liked some other guy but that guy doesn't like MC, so It's unrequited in that sense. Then the ML comes in MC's life and MC and ML fall for each other. This one is also a really good manhwa. Makes you feel sad and happy.)
https://www.mangago.me/read-manga/sahan/ (Unrequited for ML. Makes you feel sad.)
https://www.mangago.me/read-manga/scratch_1/ (I think this one is also unrequited love. Makes you feel sad. Also this one is discontinued sadly.)
For straight manhwa-
https://www.mangago.me/read-manga/july_found_by_chance/ (MC and ML are both side characters that love each other, but the author keeps separating them so It's unrequired in the sense they love each other but still can't be together. Makes you feel sad.)
https://www.mangago.me/read-manga/the_girl_from_class/ (This one was quite good. It's about the feelings of a guy towards a girl. Makes you feel sad)
This is all I can remember at the moment. I did read other Japanese mangas that also have unrequired love like Hinakure chaser and kare to kare no ijiwaru na kyori but I don't think I should recommend them to you because the ML in both of these mangas can make you more mad than feeling moved or sad. They don't even make you feel happiness...
If you read GL then this one also has unrequired love. Makes you feel sad. Not a love triangle tho. https://www.mangago.me/read-manga/only_need_to_love_once/

I always wanted to read a story where the mc is the third wheel or the side character in their love interest's relationship so I'm happy to finally find a manhwa with this plot. I have searched far and wide for this sort of storyline but it's rare to find a good manhwa with a similar premise. The ones I've read previously were Japanese mangas with few chapters (4 or 5 chapters) or a straight manhwa with side character female lead. It's delightful to find one with many chapters and lots of depth. I love the pain and sadness I feel when I read these chapters. I feel for the mc