There's nothing new it's all been done before

When I have fears that I may cease to be
Before my pen has glean'd my teeming brain,
Before high-piled books, in charactery,
Hold like rich garners the full ripen'd grain;
When I behold, upon the night's starr'd face,
Huge cloudy symbols of a high romance,
And think that I may never live to trace
Their shadows, with the magic hand of chance;
And when I feel, fair creature of an hour,
That I shall never look upon thee more,
Never have relish in the faery power
Of unreflecting love;--then on the shore
Of the wide world I stand alone, and think
Till love and fame to nothingness do sink.
So I think he's preemptively mourning the loss of three things- his unwritten thoughts, unseen/written sweeping romantic poetry, and the idea of love. I don't quite know what he's saying at the end. I think maybe that he thinks about it all, and mourns it, and then decides that it's all useless, because he's going to die anyway; this is funny, because it's a grandiloquent, sweeping, beautiful poem about how none of it [the beauty and the poetry etc] matters. I don't understand the "huge cloudy symbols of a high romance,/and think that I may never live to trace/their shadows, with the magic hand of chance". What's the magic hand of chance? At first I think maybe he's talking about beautiful things to write about, but writing is not chance. Or maybe (new thought, this just in!) that it is by chance that he sees these things in the first place. Maybe?
I'm almost ambivalent as to whether the "fair creature of an hour" actually exists. She might, but the fact that it's "of an hour" indicates falling in love with each new pretty girl as he sees her. I think this is substantiated by his saying "relish in the faery power of unreflecting love". That's always a nice nostalgic feeling to fall in love with various people, who don�t know you, or don't love you back. Or is he using "unreflecting" to mean shallow? Oh boy. That throws a monkey wrench into my theory about that. Or maybe it's love that you throw yourself into without thinking. OR maybe he's not even talking about a person. "Creature" could be a person, I've heard 'divine creature' used a lot. But- maybe it's an idea. Or something. I don't know, I think now I'm just playing around with ideas that I don't actually have.
I like that he uses iambic pentameter for this. It fits the grandiosity and self pity. And there is, I think, a large element of self pity in this- "�then on the shore/Of the wide world I stand alone, and think/Till love and fame to nothingness do sink." � which is hammered in here, what with the 'alone' bit. And the nothingness. The sad, small, alone feeling of being inconsequential and regretful about all the things you'll never do or see (the self pity comes in in a big way here, because he's prematurely mourning these things). But wait! Another thought- then here's another take on it. "�then on the shore/Of the wide world I stand alone, and think/Till love and fame to nothingness do sink." So aside from the beautiful imagery here (which is really all through this poem) maybe he's not deciding it's all useless, like I originally thought. "On the shore of the wide world" is a pretty beautiful, empowering, humbling idea. So maybe he's saying that his own personal regrets about not becoming famous or not getting the girl are inconsequential. And that he's left with the world, and that's okay. Maybe he's saying that even if he doesn't get to make a mark on it, it's still a beautiful place. Ah ha! So maybe he's saying that when he starts to feel all mopey and self indulgent, he pauses and looks at the world, and regains his balance.