What does "The worst is not So long as we can say “This is the worst.”", mean?
These lines are spoken by Edgar when, in his disguise as a poor, mad beggar, he comes upon his father, the Duke of Gloucester, who has been cruelly blinded and cast out by his enemies. Gloucester tried to help Lear in the power struggle that ensued between Lear and his two older daughters, Goneril and Regan, and this is the result of his defeat. This political struggle has destroyed family bonds and laid bare the worst traits of human nature in the likes of Regan and Goneril and their husbands. Edgar himself was forced to flee after his cunning and unscrupulous brother Edmund falsely accused him of a plot, and that is why he too is now wandering around in the wilds. The shock of suddenly seeing his father in this pitiful condition makes Edgar think that things can't possibly get any worse. But he also realises that things might yet get worse still (and in fact they do).
Edgar seems to be saying in this quote that, no matter how bad the situation is, at least he is still alive and aware of things, he is still rational and in command of himself to the point that he is able to comment on what's happening. However, in future there might come a point when things get so bad that he will no longer even be able to do this, he might not even be able to think any more.