Harry is a man of contradictions. He is brave and unsure, dismissive and thoughtful, satisfied and regretful.
At the beginning of the story, Harry is brave in the face of death. But his bravery and defiant attitude lead him to be cruel to Helen. There is a fine line between this bravery and an attitude of giving up. He is brave in accepting death, but in accepting it, he has given up. By giving up, there is no need for bravery.
This relates to how he treats Helen. On one hand, he has bravely accepted his impending death. On the other hand, in giving up, he takes on a nihilistic attitude which makes it seem okay for him to treat Helen with cruelty. Harry calls Helen a "rich bitch" but he admits to actively seeking out rich women. Harry eventually stops his poor treatment of Helen when he determines that his frustrations are not her fault.
Harry has traveled far and has had a full life, but he is haunted by his experience in the war. The main regret he repeats is that he did not write enough and in a wider sense, that he did not accomplish enough in his life. He realizes he might have been taking out his own frustrations on Helen.
Why should he blame this woman because she kept him well? He had destroyed his talent by not using it, by betrayals of himself and what he believed in, by drinking so much that he blunted the edge of his perceptions, by laziness, by sloth, and by snobbery, by pride and by prejudice, by the hook and by crook.
Among his regrets of having not written enough, he comes another conclusion, more regret: that he gave more to women who had money than he gave to women that he actually loved, "it was strange that when he did not love her at all and was lying, that he should be able to give her more for her money than when he had really loved." And in this sense, in addition to not writing enough, he feels like he had sold out.