"We went into the evening confident we had a good path to victory. I don't think there was one person who saw this coming," the unnamed senior adviser said.
After polls closed in Pennsylvania and Ohio and were won by President Barack Obama, and North Carolina and Florida were seen as unexpectedly tightly contested, "We just felt, 'Where's our path?' There wasn't one," a senior adviser told CBS News.
An aide said Romney was stoic as he spoke by telephone with Obama after the election was decided, but Romney's wife Ann cried.
Vice-presidential candidate Paul Ryan seemed genuinely shocked, and his wife Janna cried, the aide said.
After Romney delivered his hastily-written concession speech, "He was shellshocked," one adviser said.
A few hours after Romney conceded to President Obama, Romney told his staff they had just witnessed his last political campaign, The New York Times reported.
However, two people in that meeting in Boston said Romney said, "I will not fall off the map."
Friends, aides and advisers have speculated Romney could write a book, return to the financial sector or could take on a major role in the Mormon Church
"The only door that is closed to Mitt Romney for the remainder of his life is being president of the United States," said Steve Schmidt, a campaign adviser to Sen. John McCain, R-Ariz., in his unsuccessful 2008 bid to be the president. "He can do whatever else he wants to do."
Losing a presidential campaign, however, "is something you never get over," Schmidt said. "The question is whether you can move forward without bitterness or rancor."
At a breakfast for top advisers and donors Wednesday, Romney spoke of how Obama campaign turned out such huge numbers of voters, and expressed frustration about opposition attempts to characterize him as an enemy of women, which Romney said was simply untrue, those attending the event told the Times.
"He will be sifting through this for quite a while," said Kirk Jowers, a friend of the former Massachusetts governor. "The question is whether the sifting takes a couple of hours a day instead of being all consuming."