Percy is one of the most powerful people to walk the face of the earth. He is loyal to friends unto death, and would fight anyone to protect them.
He is also one of the most emotionally fragile people on the earth. After his trip into Tartarus, Percy has had a growing fear of the dark, and he sleeps as little as possible, hoping to keep the nightmares away. Sudden, loud noises startle him, and no one can do a thing to calm him down anymore (except his mom).
When he saw the flames at the alter, he is reminded of the food that saved his and Annabeth's lives in Tartarus. The creek flowing through the woods cause flashback to the River of fire.
And every time he saw a janitor, or even just a broom, he was reminded of Bob and Damsean, who'd sacrificed their lives to get Percy and Annabeth out. Every time he used his powers, he is reminded of the time when he nearly killed Ankys.
His sword reminded him of how close he'd come to dying way too many times. The haunted looks in the Camper's eyes reminded him of how Annabeth had looked as they were escaping the Pit.
Every time they burned food, he was reminded of how the Campers saved his life in Tartarus without knowing it, and how he and Annabeth had made a note on a napkin to send them a message.
Every time he saw a monster he encountered in the Pit, he see's a glimpse of their very different appearence that he only ever saw in Tartarus. He could even see different forms of monsters that he hadn't seen in Tartarus, and the Mist had stopped working for him, revealing the truth of everything.
What kept Percy going was his friends. They could see something different aout him since his trip to the Pit, even though they couldn't figure out what exactly it was. His fatal flaw kept him from leaving. It kept him in the New York area, training new demigods.
Until the day he no longer had friends to give him a reason to live.