Quatrain 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Past cure I am, now reason is past care,
And frantic mad with evermore unrest,

Just when we thought things couldn't get any worse for this poor guy, they do. Not only is he going "mad," he says he's past the point of hoping for a cure.
What the heck does that mean? The line is a shout-out to a famous proverb—"past cure, past care" (source). Basically, the proverb means that it's pointless to worry about a patient or try to treat him when he's so sick that there's no hope of a cure.
But here, Shakespeare gives the proverb a little twist. The sick patient (our speaker) says he's past cure because his doctor ("reason") doesn't care about him anymore.
In other words, he's saying that his inability to reason or think straight is making it impossible for him to get better.
Plus, he can't sleep and feels totally restless, which is driving him even more nuts (frantically "mad").

My thoughts and my discourse as madmen's are,
At random from the truth vainly expressed;

Dang. Our speaker sounds a lot like a feverish patient who has become delirious, don't you think?
He says his thoughts and his speech ("discourse") are like that of a crazy person—he's been running around saying a bunch of stuff that isn't true.
Hmm. That's interesting.
What's he been lying about lately? Has he been lying to himself or to other people? Has he been lying to us?
Has he been babbling random nonsense throughout the entire sonnet? Can we trust anything he's said up to this point? Can we believe anything he says after this?
Stay tuned.