Considering the text, the lyout makes clear it is arranged into freeverse. There are fourteen lines and the line lengths is compact in the first, second and third quatrain.

In the first quatrain, Shakespeare addresses to his beloved, telling her how much he is getting old.
He compares himself to a tree in the autumn season: like a tree whose leaves gets yellow and fall down, his hair are getting grey; and as the tree’s branches shake for the cold winter’s winds, his limbs tremble at the change of the time. His poetry becomes as "bare ruined choirs" too.
On the other hand, in the second quatrain he compares to the process that the day undergoes with the passing of the hours. He is in the "twilight of the day," the time when the sun "fadeth in the west." For him who is living the last days of his existence, the night becomes "the black night" that pass away not only his life but also take away the "the death’s second self" (his sleep); he could not sleep because the black night stole his life.
In the third quatrain, the poet introduces another metaphor: he compares his life to the fire’s life “That on the ashes of his youth doth lie”. Once the fire burned generating light, now the light is decreasing.
In the final couplet, his beloved offers him her love and this love is stronger, even if it is at the end.