"lets try and forget it... I mean ,try not to forget it... but let's forget it at the same time ". What does David want to forget ,yet not forget?

David wants to forget the ordeal with the wave after Mr. Ross unveils that his experiment was a metaphor for Nazi Germany.

He wants to forget the negative aspects of it like the differences it caused between him and the people closest to him but he doesn't want to forget the lesson it taught him about self-identity, leadership, community and fascist characteristics in modern organizations to prevent such an incident from occurring again.