Doomguy is so convinced of his life as a demon slayer that he doesn't really wish for hell to end. He doesn't have an opinion about all this either. Even if the story was over and Doomguy had to retire - he would never retire. He'd be always ready to do the only thing he knows. For Doomguy it's not a challenge. It's purpose without having to motivate himself. It's all he is. For him there is no culture shock, no inner conflict and therefore no trauma. Because he never knew anything besides hunting demons it's his clear one and only anchor at the horizon, no matter the direction he's taking. A unknown path without never getting lost.
My assumption about Doomguy's mental state is that he is like Mike Tyson in his early years when he was this furious blow of impact without any traces of doubt. Tyson himself said once that in the past he fought better because he wasn't thinking. His decline started when the thinking started.
I find Doomguy's state very fascinating! It goes beyond purpose and motivation. I think anyone who finds a "productive" calling able to commit to like Doomguy does, being a lucky fella. Additionally, if you then are able to reflect on your actions, enjoy the ride and progress of your accomplishments but still not put yourself in danger of loosing that epic strong drive by any sort of negative influences, then you are winning in life.
Be like Doomguy!