The speaker begins with a stark admission: he owns nothing, not even the spirit inside him. The title, "Poor in spirit," seems to echo that feeling of emptiness, of having no real inner possession to hold on to. From there the thoughts turn bleak and a little bitter. If trouble came, he says, he would gladly lose all his money, and he seems to think that loss would be better than keeping a mistress, because with her he is "always a devil." That suggests guilt and self-disgust, a sense that desire brings out the worst in him.
The last lines are the most striking. He drank away his wedding night, and the bed becomes "a great sea / that might not go out," like a tide that never retreats. The image suggests a gap or loneliness he could not cross. Judging by the imagery, the mood is one of regret, shame, and drifting numbness, delivered in short, plain, confessional lines.