Manga with the main character being given the power to slay dragons by his future self

Question

The main character at the beginning is very weak but his future self comes back and gives him the power to slay dragons. In the beginning his friend, who is a girl, is a hero who is very good at dragon slaying but when the main character gains his powers he leaves her.

The dragons I remember are a saintess-like dragon, in human form she can turn back time, and a barrier dragon who protects her. There's a tempest/lightning dragon that is the strongest one, and a dragon who can turn into a tree. I can't remember what it is called though.

The dragons are former dragon hunters that were turned into dragons making them hostile to humans instead of dragons. They have a god they look up to and who gives them orders. I'm pretty sure the god is represented as a little girl. The dragons try to turn the main character into a dragon too, but I don't remember if they succeed.

Answer 1

Sounds like Ragna Crimson.

From Anime-Planet:

In this action-packed dark fantasy, humanity lives under the threat of annihilation by immensely powerful dragons. The dragon hunter Ragna embarks on a revenge-fueled quest to eliminate that threat once and for all. Dragon hunters: warriors armed with special silver weapons who kill their prey for bounty. Lowest among their ranks is Ragna, who forms an improbable partnership with the young genius Leonica, a master dragon slayer with more kills to her name than almost any other. All Ragna wants is to stay by Leonica's side, but his dream is shattered by an attack from the deadliest dragon imaginable...

At the beginning of the story, the main character, Ragna, tags along with a girl named Leonica as she hunts dragons, but she's far more talented at it than he is. He later encounters an older version of himself, from a future where he's become much stronger, but too late to save Leonica from a violent death. The older version of Ragna then gives his strength to his younger self, so that he can prevent this version of the future from coming about.