When Shakespeare’s 154 sonnets were published in 1609, they were very cleverly arranged in such a way as to form a continuous narrative, like a novel. They did not have this when they were written. We do not know who made this arrangement, or whether Shakespeare was involved in the publication of his sonnets at all. Consequently, the Sonnets do not have to be read in accordance the narrative given to them by the publisher.
That said, in the collection as it stands, the implied author, the narrator as we are asked to imagine him, is a lonely, middle-aged Poet who is insecure both in love and in art, and who takes a ‘fair youth’ for the Muse of his love poetry. This rather worthless young man, about twenty, is one of those pretty boys who but for a sudden caprice of Nature (says Shakespeare) might have been a girl, and is popular, a head-turner, promiscuous and self-centred. At first, the Poet urges him to settle down and marry, but he refuses. The two unequal friends are slowly driven apart by the Youth’s vanity, betrayals (he steals the Poet’s girlfriends) and neglect. The Poet finally shakes him off in Sonnet 126, and finds a nice girl for himself in the final two poems; but in Sonnet 90 he is still not master of his own will, and is more concerned that the fair youth is thinking of dropping him.