Dave Bonta's three-line poem begins with an image most people would rather not dwell on: a single turd floating in a toilet bowl that is otherwise spotless. Then comes the comparison that gives the moment its weight. It looks like "a persistent doubt."
The simile works because of how well the two things match. Everything around the turd is clean, just as a mind might feel mostly settled and sure. But one small thing refuses to go away, and it draws all the attention. Like a doubt, it's unpleasant, hard to ignore, and stubborn about staying put. The word "persistent" suggests that flushing it away may not be as easy as it sounds.
The speaker seems tired, wry, and a little rueful rather than panicked. The humor is blunt and slightly absurd, since a lofty feeling is tied to a very ordinary bathroom scene. That contrast makes the poem funny, but it also makes the doubt feel familiar and human.