Nobody likes to admit weakness, but for the speaker of "Yet Do I Marvel," weakness is something nobody can escape. For the speaker, weakness is being unable to understand God's ways. It's not a physical weakness so much as a weakness against a force much greater than all of us: God. But what's troublesome is that God seems like a contradiction. If He's good, why all the suffering in the world? But it's this weakness, the speaker's inability to read God's mind, that leads him to the wonder captured in the turn at the end of the poem—to marvel at being a black poet is surely not a weakness, but it's the speaker's weakness in understanding God's ways that lead to his wonderment in the first place.
Pain is just weakness leaving the body, right? Uh, okay, that might help if you're Rocky Balboa training for a title fight, but if you're like the speaker, then weakness over understanding God's mysterious ways is the cause of much suffering and confusion in the world.
Despite a list of different ways we suffer in life, the speaker turns weakness over God's ways into a strength that leads him to marvel at his own identity as a black poet.