It's been practically a month since I began searching to find it again. Basically it's a manga where the main character is a soldier in the army of his kingdom, he helps the hero team to defeat the demon king.
Just after defeating the demon king he got stabbed in the back by some monster and got reborn into his younger self.
The main character is a little boy with all his memories from the past and experience as a warrior, his father is known to be the strongest warrior in the kingdom at the time, his father is guarding a forest where monsters lives.
One day, his father takes him and another boy from the village to exercise in the forest and they encounter a giant fox beast. The beast casts a spell and is stopped by the boy who talks to the beast and convince him not to destroy the whole forest.
Later on we learn that the main character, in his past life has suffered human experimentation. Forgot to say, he got a cringe but powerful spirit which has the appearence of a cringe young girl in him which attribute is dark magic. By CRINGE i mean that despite her young appearence she is very scary she gives the feelings that she can see through you, a litle bit like litle girl with red hair in "Against the gods".
So the main character goes to military/magic school because he met another litle girl which happened to be the headmaster of a magic academy.
And in the last chapters I can remember, the main character and his group of friend are send to the vanguard of the forest his father is guarding for training.
The little sister of his best friend got lost in the forest, so everyone is looking for her. They find her, but also find a dragon.
After getting almost killed by the dragon his best friend also gets his personal litle girm spirit within him which heals him.
They go into some strange door and see the memories of the past life of the main character when for exemple a group of ravaged his village killed the girl he loved and when his best friend tried to kill a high demon he got beheaded (his head got cut).
When his best friends which is with him at the moment sees his memories,he understands the situation.
They go out of the ''cave''i would say, and are confronted with the drake.
This is as far as i read.