This is spoken to enemies who are gloating over the writer’s downfall. And instead of pretending he did not fall, he owns it and then defies it.
Though I fall, I will rise again. He does not say I did not fall. He says the fall is not the end of the sentence. Failure got a comma, not a period.
Though I sit in darkness, the Lord will be my light. Notice the light is not his own grit or a better mood. It is the Lord. He does not have to manufacture the way out. He has to look to the One who is the way out.
So say it over your own worst moment tonight. This is not where the story ends. I will rise, not because I am strong, but because the Lord will be my light.