Lear tells the disguised Kent to deliver a letter to Regan informing her that he's about to show up at her place. (Yep, that makes two letters that are en route to Regan.)
The Fool cracks some bizarre jokes, mostly about the wild ingratitude of Goneril and the fact that Lear's hope of escaping to Regan's loving arms is stupid, because Regan is likely as bad as Goneril.
Lear half-listens to him, but he can't get his mind off his one good daughter, Cordelia, who he seems to remember all of a sudden.
"I did her wrong," Lear admits quietly.
The Fool continues with the jokes. His most pointed wisecrack is that Lear should be beaten for being old before his time. Lear is all, "Huh?", and the Fool points out that men should be wise before they get old. Translation: Lear has been acting like a foolish old man, not a wise old man.
Lear is afraid he's getting senile and says, "O let me not be mad, not mad, sweet heaven! I would not be mad," which is a really subtle hint from Shakespeare that just maybe, Lear might be driven to madness.