While it may appear as if Toph’s experience has made her especially cynical in her old age, it’s very possible that Toph is exaggerating in order to break Korra’s perception of herself. Korra connects her personal trauma with her role as the Avatar, and she’s convinced herself that she’s only going to get hurt if she tries to be the savior of the world again. Toph is there to humble Korra and show her that she’s a part of something bigger, and it’s no coincidence that Toph is connected to the entire world through the roots of the swamp. Korra needs to get out of her head and find that connection to the rest of the world, because then she won’t be fighting this battle on her own. (Korra also needs to get rid of the metal poison that remains in her body, but she can only do that once she finds mental calm.)