The speaker begins by comparing his "love" to a "fever." (Translation: someone's got this dude all hot and bothered.) He says the fever's not getting any better because it's "feeding" on the thing that makes it worse. (Dang. It sounds like our lovesick—or maybe lustsick—speaker is saying his sexual appetite is unhealthy and out of control.)
Next, he compares his ability to "reason" to a doctor who's been trying to help cure him of his disease. (Does this guy love metaphors, or what?) Problem is, this "physician" is totally mad at him and won't help him anymore because the speaker's a lousy patient and doesn't follow medical advice. (Hmm. Sounds like somebody is feeling guilty because his lust is a lot stronger than his ability to act like a rational, thinking person.)
The speaker says he's super desperate now because he knows desire is fatal, especially since he didn't listen to his doctor's advice about avoiding that thing (sex) that's making him sick. Oh, boy. Now we're beginning to wonder if this "disease" is more than just a metaphor for his unhealthy sexual desire. Because it kind of sounds like he may have an STD.
At this point, the speaker starts to sound as delirious as a feverish patient. He says he's past the point of hoping for a cure because he's lost his mind. He can't get any sleep at night and his thoughts are always racing. Plus, he's been talking like a madman and raving on and on about a bunch of stuff that's totally untrue.
Then, out of nowhere, the dude stops complaining and slings some nasty accusations at his mistress. He tells her that he's been going on and on like a crazy person about how beautiful and honest and good she is. But it turns out that she's ugly and nasty and totally corrupt. Uh-oh. Sounds like someone got cheated on.