The final lines of the poem evoke a mood of somber acceptance.
In the rest of the poem, Keats is expressing anxiety about dying too soon. By the end of the poem, however, he seems to come to terms with the idea.
At the poem's end, the speaker is standing alone and thinking. He realizes that love and fame are nothing. Therefore, it doesn't matter if he dies too soon, because after he is dead love and fame won't matter anyway.
At the end of the poem, the speaker is contemplative, somber, accepting, and even resigned.