A tale of a warrior-type protagonist who learns to master himself?

Can anyone recommend a good book that tells the tale of a ninja who learns to master himself? I’m looking for something that inspires the reader to overcome their own negative emotions (fear, lust, conceit, etc.) and gain control of themselves. Mind over matter type of stuff. I’m hoping the book can explore the ninja’s thoughts to himself as he overcomes the hardships he faces and learns patience, humility, forgiveness, etc.

Like maybe during his journey he gets imprisoned and despite his hunger he refuses to eat the food given to him by the guards because he knows it’s poisoned. And then the book will detail his thoughts as he overcomes his temptation for food with his self-discipline.

Or maybe the protagonist was always someone who was fat and lazy and would ditch training sessions but when his father is killed he must develop discipline and train his mind and body to become a warrior that will avenge his father.

Ideally, the protagonist is a lone wolf with no love interest in the story. I would also prefer if there weren’t any religious, spiritual, or supernatural elements involved.

Any suggestions are highly appreciated, thank you.