The structure of this sonnet is a bit confusing because it does not fit exactly into the usual pattern of a sonnet. Traditionally there are two major types of sonnet: the Petrarchan/Italian, and the English, also known as the Shakespearean (although there are other variants, such as the Spenserian). Yes, the Petrarchan is usually divided into an octave (first eight lines) and sestet (concluding six lines) but this is not just a matter of rhyme. The change from octave to sestet also signals a change of direction for the poem as a whole. The octave introduces and develops an idea, situation or proble; the sestet provides some sort of resolution. The English sonnet, on the other hand, normally devotes twelve lines to the introduction and development of an idea then comments on it in some way with a concluding rhyming couplet.
'To Wordsworth' essentially follows the English form. Twelve lines of the poem introduce and develops the idea and image of Wordsworth as a great poet, but the concluding two lines then reproach him for having fallen away from his greatness as an artist. Shelley thus laments that Wordsworth has changed from a revolutionary ardent poet into a dull and conservative figure. The misleading thing is that Shelley has chosen to put the rhyming couplet nearer to the middle of the poem instead of at the end. Thus, instead of the more usual English/Shakespearan rhyme scheme abab cdcd efef gg we have here abab cdcd ee fgfg.