not really a manga I guess but it does make me questions life when I read and thinking hard
https://www.mangago.me/read-manga/dam_of_the_forest/
Monster by urasawa naoki I originally watch it but later on I also read the manga.
https://www.mangago.me/read-manga/echoes_of_footsteps/ but it's a manhwa :(
https://www.mangago.me/read-manga/hoshi_mamoru_inu/
https://www.mangago.me/read-manga/my_broken_mariko/
https://www.mangago.me/read-manga/solanin/
https://www.mangago.me/read-manga/bakuon_rettou/
https://www.mangago.me/read-manga/yokohama_kaidashi_kikou/
https://www.mangago.me/read-manga/nora_to_zassou/
https://www.mangago.me/read-manga/omoide_emanon/
https://www.mangago.me/read-manga/yomawari_sensei/
https://www.mangago.me/read-manga/bokutachi_ga_yarimashita/
https://www.mangago.me/read-manga/ginga_no_shinanai_kodomotachi_he/
Any recommendations (Manga) which will make you question about life and make you feel empty bitter sweet type manga