Aziraphale knew what it felt like to think oneself a failure and he was going to have none of that from her. But firstly:

"Would you like to know a secret?" he asked, smiling conspiratorially and leaning in closer, hoping to be encouraging and uplifting. "I don't believe in accidental existences. If you're here, you were meant to be here, you were meant to come into being--even if the reasons seem unclear or nonexistent right now." Ineffable. The word on the tip of his tongue is ineffable.

"Besides," he continued. "Sometimes one fails at what they thought their original purpose was, only to discover they are actually extraordinary at something entirely different."