I see that in DWM 487, Steven Moffat says that the Curator is meant to be a future Doctor and that the Eleventh Doctor would not remember the specifics of his conversation with the Curator but will leave the Under-Gallery "with the strange, groundless conviction that Gallifrey is still out there". If so, this would explain why in Time of the Doctor he still believed he would die on Trenzalore. Since he doesn't remember the encounter with his future self, he had no way of knowing that he would survive.