If reality shows had been around back in the 16th century, our speaker probably would have starred in an episode of Cheaters. Can't you just picture him confronting his girlfriend and accusing her of being a promiscuous liar? The surprising thing about Sonnet 147 is that the theme of deception doesn't show up until the final lines of the poem. That's when the speaker mentally kicks himself for thinking his mistress is a better person than she actually is and hurls some nasty accusations at her. Come to think of it, the theme and the accusation come out of nowhere, which makes us feel like we've been duped by the speaker.
The speaker's not just angry at his mistress for being morally corrupt—he's mad that he fooled himself into thinking she was honest and moral.
It's hard to trust anything the speaker says in this sonnet—after all, the guy claims that he's gone "mad" and can't think rationally.