My thoughts and my discourse as madmen's are,At random from the truth vainly expressed; (11-12)
This is where our speaker says he's been running around like a madman—thinking and saying a bunch of stuff that's really far from the truth. So what the heck has he been lying about? Who has he been lying to? Can we trust anything this dude says? Guess we'll have to keep reading to find out…
For I have sworn thee fair, and thought thee bright, Who art as black as hell, as dark as night. (13-14)
Oh. We get it. He's saying that he's been running around swearing up and down that his mistress is "fair" and "bright" (pretty, honest, and moral). But now he knows the truth, which is that she's ugly and morally corrupt. Clearly, the guy feels like he's been duped. Given the context of the poem (it's all about the speaker's lust and desire), it's not unreasonable to assume that she's cheated on him or betrayed him in some other way. But he also seems like he's mad at himself for believing in her in the first place. So technically, we're dealing with two kinds of deception here.