The speaker of the poem is Keats himself. He expresses fear that he may die young. He is afraid that he will die before he is able to reach his full potential as a poet or, “before my pen has glean’d my teeming brain.” He makes an analogy between his production of poetry and the harvesting of grain. He wants to live long enough to create high piles of books. Here, “charactery” means printing or writing. His imagination is the grain and he, the poet, is the harvester. He wants to live long enough to write (harvest) all his best thoughts but not until they are ripe. Note the use of alliteration with g’s and r’s to combine this analogy.
The speaker then worries that he may not live to trace (write) the shadows of the symbols of romance. These shadows and symbols are his Romantic and Idealistic perceptions of nature and the world. The speaker clearly is thinking as a poet who wants to fulfill his potential. He wants fame, to be loved and admired. He, at least, wants his poetry to be remembered after he is gone.
While deriving inspiration from nature, he is also inspired and loved by his beloved, whom he calls “fair creature of an hour” to illustrate the fleeting nature of love and life (an hour). The speaker is concerned he may not have enough time for poetry, fame and love.
After considering all of this, he stands alone on the shore. The shore represents a border between two worlds: land and sea. This is analogous to the border between two other worlds: life and death/afterlife. He seems to conclude that, in the grand scheme of things, his fears of losing love and fame are not as important as he thought.
It is important to note that he ends up standing alone: apart from the reaches of love and fame. He has some kind of epiphany but the exact implications of that epiphany are ambiguous. He wants to sink his fears and be less concerned with fame. It may just be that, in contemplating his own death, he realizes how insignificant fame and “unreflecting” love are.
It is a lonely conclusion but the point is to consider what is important in life by separating yourself from it. Surely fame is insignificant, but what about love? I think the love he’s talking about here is receiving love. He uses “unreflected” because this is love he receives from his beloved. In this context, he uses that love as poetic inspiration which, as he previously stated, was in the name of fame: not love. This may be part of his epiphany but that is subject to each reader’s interpretation. Keats died young. This poem is definitely about his desire to be, and be known as, a great poet.