You could do it like regular dialogue between two people. The "real" character could give his better self some kind of snarky nickname which you could use as a dialogue tag.
Jon sneered as his other self. "Why don't you shut up, Angel Fart. I stopped believing in virtue and nobility years ago."
"If you stopped believing, why am I here?"
Or you could do it like normal internal monologue but with the good Jon’s comments underlined/italics.
Jon fought to ignore his inner voice. He knew what he had to do, and he'd do it. He'd stopped believing in doing the right thing years ago.
If you stopped believing, why can you hear me?
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