How does Amir still carry around his past mistakes?

Amir carries his past around like the preverbal albatross around his neck. Hassan's rape, in particular, haunts him every day. Amir wishes he had done something. The scene when Assef the socio-path is beating Amir when Amir had returned to fetch Sohrab is especially poignant. As Assef savagely beats Amir, Amir is laughing. The pain and the blood are a sort of penance for being such a coward when he was a child.