Why does Amir say that the last time he sacrificed for Baba he damned himself?

The incident that Amir speaks of when he says, "the last time I had done that, I damned myself," is when he allowed Hassan to be attacked and tortured without doing anything about it. He stood by and watched as all of that happened and then he ran away. Part of the reason that he did so was because he needed to save face. He couldn't allow his father to know that he didn't win the kite running event or that the kite had been stolen. Amir's inaction caused him years of guilt and regret and arguably ruined his life, thus damning him. At the point you're talking about, Amir has an opportunity to ease the guilt a little bit and save face in front of his father. It's too little too late, but that's what he does.

source