My thoughts and my discourse as madmen's are,At random from the truth vainly expressed; (11-12)
Now this is interesting, Shmoopers. The speaker certainly wants us to believe he's losing his mind, but,he doesn't come out and say that he is mad. He says his thoughts and speech are as madmen's are. In other words, he's acting like madman because he's been running around saying a bunch of stuff that isn't true.
For I have sworn thee fair, and thought thee bright,Who art as black as hell, as dark as night. (13-14)
So this is supposed to be our speaker's proof that he's been talking and thinking like a madman. He thought his mistress was beautiful and moral, but he was crazy to think that because it turns out that she's not. The problem is, this isn't evidence that the guy is mad. We think it's just the opposite. He knows the truth about his mistress—otherwise he wouldn't confront her here. Just like he's known all along that his desire for her is unhealthy but he can't control it.