Nathan couldn't fully acclimate to his life now because he was waiting for the other shoe to drop. For something horrible to happen that he totally could have prevented if his powers weren't off.
Seeing minor injuries and accidents happen to the people he loved was hard enough but that wasn't his biggest concern. It was that something irreversible could happen if he wasn't able to stop it. Something with permanent consequences.
How many people had he stopped from dying over the years? He hadn't kept count but he knew it was a lot, especially since his visions began showing more dangerous things.
The Ferris wheel incident may have been dramatic but it wasn't the first time Nathan saved someone's life. It may have been the most people he saved at once to date though, even now. Either that or the fire that ended his moonlight career.
