https://www.mangago.zone/read-manga/semantic_error/
it's bl, the mc is not exactly gloomy but he is a loner one of my favs!
Perhaps some of these
https://www.mangago.zone/read-manga/ijime_kko_ni_fukush/
https://www.mangago.zone/read-manga/tonari_no_metaller_san/
https://www.mangago.zone/read-manga/gozou_roppu_no_koi/
https://www.mangago.zone/read-manga/isso_koe_ga_nakattara/
https://www.mangago.zone/read-manga/sex_drop/
https://www.mangago.zone/read-manga/a_death_wish_and_an_alien/
https://www.mangago.zone/read-manga/mask_danshi_wa_koishitakunai_no_ni/
https://www.mangago.zone/read-manga/sonna_me_de_mite_kure/
https://www.mangago.zone/read-manga/bokura_no_tsuzuki/
https://www.mangago.zone/read-manga/wagging_the_tail/
romance with a very gloomy/loner character? can be bl or shoujo or yuri