Littlemissfg

Hey, I have been stuck on finding this certain anime and it is driving me up the wall. In August/September 2019 I came across an anime ad on Facebook, clicked on it and watched about 10 mins of it. I was fairly into it. Anyway long story short I accidentally clicked out of the window and I couldnt find it again. The start of it was an old man (his back to the camera) hes writing down letters talking about his childhood. It jumps to him in 1942/1943 and hes at school and sees this girl. This girl doesnt have much money and has a little sister but her mum is unwell and she gives her a fruit of some kind. She claims she wants to become (I think) a teacher and the guy has fallen in love with her. It jumps to him being a soldier ( or training to be one) he wore a cap and luckily he is away from where the Hiroshima bomb goes off. He runs over to it and theres injured and rubble but also he thinks the girl he is in love with is under the rubble. ( and that's all I watched) If you or anyone else knows what anime this is, it would be very much appreciated.