It's from a fragment of a poem he never finished, which his wife published under the title "To the moon" The words are slightly different from what you remember: Art thou pale for weariness Of climbing heaven and gazing on the earth, Wandering companionless Among the stars that have a different birth, And ever changing, like a Joyless eye That finds no object worth its constancy?
How did you get on with Ozymandias?
Actually, I had a similar difficulty when I encountered Shelley in middle school - the problem was that his poems are long, and definitely not modern in tone, which means it is often hard work at the start.