In Henley's "Invictus," the use of night primarily symbolizes the struggles, challenges, and hardships the speaker faces throughout life. However, it does not specifically represent sadness, pain, stress about mistakes, grief over a loved one, or fear of an uncertain future in a direct sense. Instead, night serves as a metaphor for the darker moments and adversities one encounters. The poem ultimately emphasizes resilience and strength in the face of these difficulties.
Given the options you've provided, the first one— "It symbolizes the sadness and pain the speaker feels during difficult times"—could be considered the most fitting response, as night often represents the struggles and emotional turmoil of the speaker. However, it is important to recognize that the overarching theme of the poem is one of perseverance and self-mastery, indicating that despite the darkness of night, the speaker refuses to be defeated.