The first stanza shows the speaker of the poem attempting to come to grips with the fact that his lover apparently no longer wishes to continue their relationship. He also displays his gratitude to her for offering him a kind of hope, and he wishes now that she would consent to go on one last ride with him (literally, a ride on horses, though "ride" also can be read as having a sexual meaning).
In the second stanza, the speaker watches as his mistress considers his request. He feels that her answer is a matter of life and death to him; if she consents, he lives; if not, he dies.
The third stanza seems to suggest that being with his mistress feels like heaven to the speaker. When they are together, it feels so heavenly to him that flesh seems to disappear. The references to clouds possessing "billowy-bosom[s]" and to "passion" and "flesh" all hint at the speaker's lust for his mistress. When she leans forward to him, he feels both joy that she might consent and fear that she might refuse his request. Evidently, she accepts.
In the fourth stanza, the speaker and his mistress actually begin their ride. He compares his soul to "a long-cramp’d scroll / Freshening and fluttering in the wind" because he is so happy and free in this present moment. He tells himself not to worry about the past and what might be different now if he behaved differently then. He tries just to focus on the here and now: their last ride together.
In the fifth stanza, the speaker consoles himself that he is not the only man to ever try and fail to achieve something he wants. He is so happy during this ride with his mistress that he feels as though his "spirit [flies]." The narrator hoped she would love him, and though she does not, at least they have their ride together.
In the sixth stanza, the speaker asserts that his last ride is better and accomplishes more, than a statesman or a soldier could during his entire life.
In the seventh stanza, the speaker tells the poet that he can only tell about, or "beat into rhythm," what the lover actually feels. The poet spends his time putting experiences into words. In so doing, the speaker wonders if he actually experiences everything he could or should. The poet is no closer to experiencing the "sublime" than anyone else, but the speaker feels he experiences it in his ride.
In the eighth stanza, the narrator speaks to the sculptor who gives his years to his art and yet never experiences the feelings that the speaker does on his ride. It’s the same for the musician who grows old while he composes and yet never feels the joy of such a ride.
In the penultimate stanza, the speaker considers the idea that if he successfully won the love of his mistress, life on earth would be better than life in heaven. Since he is unsuccessful, both life in heaven and with his mistress are still even better than their ride.
Finally, in the last stanza, the speaker dreams that this near-perfect ride could just go on forever. This moment is his "life’s best," and he imagines what it would be like if they could just ride on into "eternity."