The poet starts talking about himself in the fifth line.
Explanation:
The fly is a poem by English poet William Blake, and it was published in 1794.
In the poem, the first stanza (that includes 4 lines) talks about the fly and sets the time of the year (summer.) After that, the last 3 stanzas talk about the poet and how he contrasts his own life with the fly, which is literally mentioned in the last stanza when the speaker says "Then Am I/A happy fly."
In the fly, blake marks a change of voice in line 5- while in the first stanza, he considers the fly itself and its action, in the following stanzas, he reflects on his own life
