It's a doujinshi meaning it's self published. I think this was one of the earlier manga sensei wrote
Yes I understand that but I don't know which is the original piece, cause I read through some of the author's book but I can't seem to find the original one
It doesn't, because it is the original one. If I remember correctly, this is just a one shot from a collection of short stories sensei made in the earlier years. Doujinshi isn't always derived from an original/existing work and can just mean it's something authors produce independently on their own. Hope this helps!
Yes, I just found out yesterday that this is the original, it's from a manga call erm I don't remember but it is a one shot compilation of Ogawa sensei Thank you a lot
It comes from the oneshot series Sekai wa Kimi de Mawatteru chapter 7
What was the original piece if this was a doujinshi ?