I found the main theme to be the dangers of complacency and the fear to make a change. In this specific case, he is trying and failing to get the courage to ask out a woman. He keeps trying to wait for the perfect moment or phrase (“And for a hundred visions and revisions,\ Before the taking of a toast and tea.”). Each time, he wonders if this will be the time he gets the courage (“To wonder, ‘Do I dare?, and, ‘Do I dare?'”) but he has fallen into complacency and lacks the courage, and now he is growing old (“With a bald spot in the middle of my hair”).
He is also aware that the older he grows the less likely he will be to succeed (“I have heard the mermaids singing, each to each.\ I do no think that they will sing to me.”). He wishes he could start over and try again, comparing himself to Lazarus being brought back from the dead (“To say: “I am Lazarus, come from the dead”).
Finally, he connects this complacency and cowardice to the rest of his life, where he feels he was nothing but a side character (“No! I am no Prince Hamlet, nor was meant to be;\ Am an attendant lord”). He realizes he was nothing but a creature of habit (“Have known the evenings, mornings, afternoons,\ I have measured out my life with coffee spoons”) but that it is now too late for him to make a change (“And I have seen the enternal Footman hold my coar, and snicker”).