At the beginning of the poem, the speaker states that it's about time he shouldn't be falling in love anymore. He considers his age and says that nobody will fall in love with him, but this doesn't change the fact that he wants to fall in love. He describes himself as being in the "yellow leaf" of his life, meaning he is essentially in the "autumn" of his life. He admits that his youthful days are now over, and he is only left with decay and old age.
The speaker then expresses his feelings of loneliness, comparing his heart to a lonely "volcanic isle." He thinks back on how powerful love used to be and the intensity of the emotions he used to feel. Now, he does not have those kinds of experiences and lives a very different life.
However, the speaker states that these things should not be troubling him now, as he is preparing to go to war in Greece. He tries to repress his longings and problems, saying that he should not think about those things anymore as he has more honorable things to be concerned with. In the final stanzas, the speaker expresses his feelings of regret, stating that if a person looks back on their life with regret, what is the point of living? Finally, the speaker tells us he desires to die for an honorable cause.
