I personally feel John 100% believes his lies. Some might disagree and say it's an act, but he's just been too persistent with it for so long, day in day out, lie after lie after lie, for years. I don't think anyone could keep up an act for that long.
John is the type of habitual liar who has crossed over into a higher plane of lying. At this point his entire existence is composed of lies. He has no choice but to just keep doubling down, piling the lies higher and higher. They form a protective shield around him, and that's why he reacts so viciously to anyone challenging him on what he says. It's not just a disagreement to him, its an attack on the foundation of this person he has created.
He also has something in common with Zoe "Chelsea van Valkenburg" Quinn: The lies are the bodyguard around the ugly truth and what they need to keep themselves from realizing the trap they placed themselves in.
Both Zoe and Brianna know that if no one believed their lies, they'd be worthless not only to the world, but to themselves. Neither has ever achieved anything meaningful with their own blood, sweat, and tears. They have skated by on other people's money and talent and have managed to coast by thus far using that as a springboard to attention whore themselves into the spotlight they crave, and if it wasn't for the fact they had the dumb luck to strike when they did, it would have been just another failure in a life full of them attempting to achieve glory using their own gifts.
And in that they have trapped themselves. The lies they have proclaimed to the world for so long and so loudly cannot be recanted, cannot be admitted false, cannot be walked back on, and they know it all too well. If they ever admitted to the world what they refuse to admit to themselves, they would have to look in the mirror and see someone with no integrity, no skill, and no ability to achieve anything based on their own effort.
And both would rather die than do that, because if their lies crumbled, it will be the end, even if it doesn't end in literal death, they will be dead to the world except as possibly an object of contempt.
And that thought fills both with such horror they will delude themselves to the end in hopes it will never happen.