My reason, the physician to my love,[...]Hath left me [...] (5-7)
Why on earth would our speaker talk about his ability to reason (think rationally) as if it were a person? (A doctor, to be exact.) It seems like he wants to admit that he's unable to think straight and make rational decisions because his desire is so powerful and out of control. At the same time, he also seems unwilling to take responsibility for his condition (and possibly his actions).
Past cure I am, now reason is past care, (9)
Here, our speaker claims that he's got no hope of ever being cured of his lust-sickness because reason has given up on him and is "past care." Do we buy it? It seems like the dude is just making excuses because he doesn't care to listen to reason.
And frantic mad with evermore unrest, (10)
Here's where the speaker starts to actually sound a little delirious. (Kind of like a feverish patient who's restless and out of breath.)