Who is the antagonist in The Narrow Road To The Deep North?

The novel relates events at different points in Dorrigo Evans's life. At each stage, different antagonists are presented and Evans must struggle to overcome the problems they present. Because the action spans several decades, including Evans's ordeals as a World War II prisoner of war and peacetime roles as a doctor, any of those antagonists might be considered central. Another character, Major Nakamura, occupies a prominent role later in the novel.
Because the novel covers a long period of time and shows Evans moving in and out of so many situations, it lends support to a central, more abstract antagonist: the power of the past. The introduction of Nakamura's story toward the end supports this idea.
While an Australian soldier, Dorrigo was taken prisoner, serving as a physician in the POW camp for laborers on the infamous Burma railway. Much of the novel recalls the horrific conditions and the strained relations between prisoners and Japanese and Korean captors. Because of the emphasis on this situation, including as part of Evans' later reflections, the enemy soldiers could be considered the antagonist. However, the complexity of the interactions presented makes it likely this is a secondary-level conflict.
During the war and after, Dorrigo's personal struggles, including complicated affairs of the heart, keep him looking backward. As an older man reflecting on the past, and finding difficulty resolving his inner conflicts, the past weighs heavily on him. Chased by demons he can never quite escape, he attempts to rewrite the past as he composes an introduction to another man's record of the war. Those ongoing struggles give weight to the idea that the impossibility of escaping the burden of the past is the greatest opponent that Evans confronts.
https://www.theguardian.com/books/2015/apr/22/the-narrow-road-to-the-deep-north-review-richard-flanagan