In this scene, Edgar (pretending to be a poor mad beggar) meets with his blinded father. His father doesn't recognise him and assumes it's the local fool, Poor Tom (Edgar's disguise).
Edgar has the stress of meeting his father again, and almost being found out. But because Edgar is a smart cookie, he weighs up the situation and realises the worst that he can imagine is not the worst that can happen.
The worst possibility is that his disguise is found out, by his father. He works out this won't happen in reality. So if he says, "this is the worst thing that has happened", it is not the worst he can imagine.
Often what happens in real life is never as bad as what we think. So we can say, "The worst is not, So long as we can say, 'This is the worst.'"