Robb didn't take your head when he woke from stasis. If he couldn't forgive you, he would have killed you.

[It had been frustrating then, before Jon's memories were fully returned. He could understand now why Robb had left Theon be, even if it meant his usual japes and cruel comments. The truth was, Robb loved Theon like a brother. No matter his crimes, he would be forgiven...in time.]

That is your punishment, Theon. Being killed ends it all quickly. There's nothing to haunt you or think about once you're dead. It's living with it that's hard. You failed, but go forward and fail again. Atone for the things you did by becoming a better man.