@AnceldeLambert They establish quite early on that he was motivated by pure rage, vengeance, and the expectations of his fallen friends and family.
It's a bit hard to believe but a person who endures many hardships but has a clear goal in mind is able to concentrate on progression and ignore obstacles. All he had to do was practice and mine. Both of those were time consuming enough to allow him to stay sane. Focused on a single outcome. Void of any reasoning besides moving forward. He also didn't have to worry about being tired or hungry because poison water and poison grass gave him energy. He could work 24/7.
It just goes to show how important determination and discipline is in the endurance and overcoming of hardship.
Also, this happens in other Murim stories like Chronicles of the Heavenly Demon or Volcanic Age, where cultivators go into seclusion and training for up to a decade. The only difference is those forms of seclusion usually integrate voluntary practice of patience, discipline and progression while Jagang's seclusion was forced upon him and required him to make progress and avenge his people or die in silence and disappointment. It was a do or die situation and he worked hard so he wouldn't die.