If the two of them could just harness their guilt as a productive energy, they could probably topple the newly-instated Galactic Empire all by themselves. And Obi-Wan, at this point in time, hears not what she's trying to convey, but a recrimination, however unintentional (and he's aware if Padmé wanted to rebuke him, she would not couch it so nicely): I didn't trust you to keep Anakin safe during the war. His own guilt puts meaning to her words she didn't intend, and only feeds itself. He was self-centered, arrogant, and didn't pay attention to his apprentice-turned-comrade, his brother-in-arms.

The questioning of previous actions and inaction is nothing new to him now, and Obi-Wan tries to push it back; he will return to these thoughts, he knows, but now is not the time. He gives Padmé a bittersweet smile. "I assure you, m'lady, I will be all right. I appreciate your concern, but don't worry about me overmuch. If this whole ordeal has proved anything, it's that I am, above all else, a survivor."

It's not a trait he's particularly proud of, since it inherently means there were others who didn't survive, whom he couldn't save.