It is not upon you alone the dark patches fall, The dark threw patches down upon me also, The best I had done seemed to me blank and suspicious, My great thoughts, as I supposed them, were they not in reality meagre? would not people laugh at me?
As the sun sets, the poem continues to get darker. Now the shadows are falling on the speaker, and he interprets these "dark patches" metaphorically as dark thoughts.
One of these thoughts is to question his achievements as essentially empty, or "blank and suspicious." He worries that people will laugh at his thoughts. This, surely, is an experience that everyone can recognize.