Hi Nocturne.
This nocturne is in ternary form. The contrasting section B starting at bar 19 in my copy changes the mood and becomes first tranquil but then angry and passionate at bar 27.
The purpose of bars 31 - 43 is to manage the transition back to the return of section A at 44. Play it softly, gradually slowing until the adagio at bar 42.
Understanding what this nocturne is all about helps here. Although it was published after his death, Chopin actually composed it in 1830. It is one of a group of nocturnes he did not allow to be published because he thought they were of insufficient quality.
In 1830, Chopin was 19. Poland had been invaded and carved up between the Russian, Prussian and Austrian empires. The conquering powers embarked on a ruthless campain to destroy Polish culture; their language was banned and schools closed. Leading artists, writers and musicians were murdered. The Russians were especially brutal anbd Chopin's home lay within their territory.
Chopin fled to Paris at the age of, I think, 17, leaving his family behind. Imagine the feelings of a young man chased from his home and arriving friendless in a foreign capital city. These posthumous nocturnes bear all the signs of a young man yearning for his homeland and family (section A) mixed with bouts of calm acceptance and cries of anger (section

.
Be aware of all this as you play the nocturne and you cannot go too far wrong.
Incidentally, here is a thought for you. Such a brilliant musician was welcome at the courts of the aristocracy. He paid his way by performing for them, often improvising. He improvised most of the nocturnes at these soirees first, then went home to write them down.
It takes me months to learn to play one of them. Chopin just sat down and made them up as he went along.
Ye Gods.
Hope that helps.
Steve