“If Neville is successful, the boggart is likely to shift his attention to each of us in turn,” said Professor Lupin.
“I would like all of you to take a moment now to think of the thing that scares you most, and imagine how you might force it to look comical. . . .”
The room went quiet.
Harry thought . . . What scared him most in the world? Rowling, J.K. Harry Potter and the Prisoner of Azkaban (p. 54). Pottermore Publishing. Kindle Edition.

Second thought coming right on top of that one was, ‘Do I really want to show it to the world at large?’ With the one right after that was the old saying and proverb, ‘Know yourself.’ Combining them together to Knowing what scared him, but letting others know.

Did Voldy scare him?… Not so much him, as what his followers represent to his friends and the world. And the fact that he seems to have been led out as this generation’s Judas Goat. Like his parents before him. And he didn’t think a boggart could enact a true concept, so that was out.

Dementors? Those Soul Stealing Demons? To quote FDR, ‘So, first of all, let me assert my firm belief that the only thing we have to fear is fear itself—nameless, unreasoning, unjustified terror which paralyzes needed efforts to convert retreat into advance.’ And fearing fear demons, is not the way to go. To defeat them is to defeat the fear of them. And, as they represent death. A death I have been faced with multiple times in these last few years, it holds nothing over me anymore. If they eat my soul to create, in me, another of them, I shall have been destroyed and it won’t matter anymore. The fear of death no longer holds its deathly hands over me.

He noticed it had gotten quiet in the room, and everyone was looking at him. Professor Lupin was looking uncomfortable as he came forward. And when Lupin didn’t move, he opened the, now, quiet wardrobe doors. And nothing came out but stayed inside the wardrobe. In the darkness. “First of all, let me assert my firm belief that the only thing we have to fear is fear itself—nameless, unreasoning, unjustified terror which paralyzes needed efforts to convert retreat into advance.” A keening noise came from inside, and the wardrobe shook as he pointed his wand at the darkness inside.
“Begone nameless, unreasoning, unjustified terror. Begone into the depths of hell.”
A high-pitched wail, then a bright light. And a scent of rose petals filled the, now, empty wardrobe as the boggart was banished.

He turned to the shocked professor, bowed his head, “Your wish for me to show my fear to the world will have to wait for another time professor. But I thank you for getting me to inspect my inner self for it. However, time is up. Any homework?”