sealed even from himself
Like most North Americans of his generation, Hal tends to know way less about why he feels certain ways about the objects and pursuits he's devoted to than he does about the objects and pursuits themselves.
Context
The innermost version of the theme: the boy who cannot make himself understood to a room cannot make himself understood to himself either. He broods on the No-One-Must-Know compulsion and the brooding leads nowhere, because he knows the objects of his devotion intimately and their causes not at all. The wall runs all the way in, so that the self is a figurant to its own interior.