I don't understand why everyone thinks Percy and Annabeth are so broken after Tartarus. They actually got off pretty fine. Despite all the death and horror they saw, they survived. They had something to eat and drink, and they even had help from two extremely powerful beings. I understand the loss of Bob and Damasen, but they still got off okay.
Nico, on the other hand, went through it alone, probably never drank from the Phlegathon because he had the pomegranate seeds, and he didn't have anyone but himself. He probably had a little control over the spirits. Also, even as he was getting out, he got captured. Plus, Percy and Annabeth are about 17, and seen a whole lot more, Nico is 14 (minus the Lotus years.) Imagine how difficult it must have been.
See, when Percy and Annabeth are finally ready to talk about Tartarus, and everyone is comforting them; imagine Nico's dilemma, he went through worse and nobody is nearly as worried about him. I feel sorry for Nico. Any and all fanfics I will write in the future will focus on Nico's pain more than the others.