Try this?sorry I can only remember this two..there are a lot but I don't remember all of them ╥﹏╥
https://www.mangago.zone/read-manga/life_senjou_no_bokura/
https://www.mangago.zone/read-manga/life_senjou_no_bokura/
Based on your description, this one is the first one that came to mind: https://www.mangago.zone/read-manga/will_you_marry_me_again_if_you_are_reborn/
I also really enjoyed “life senjou no bokura” which Chimyy.T^^ recommended.
Sound interesting story....I would like to read it later too (=・ω・=)
Can anyone give me really sad stories. I like wartime one specifically but anything else will do.