The East Anglians were six minutes away from a famous result despite having two men sent off against Billy Davies's resurgent Forest side before Lewis McGugan struck a crucial winning goal in the Reds' fight for a play-off place.
"We were unlucky not to hold out," said McCarthy, who had no real argument against either red card.
"It's a deflection that's cost us - they didn't look like they were going to get it any other way."
Forest's struggle to find the vital goal occupied Davies's mind after the game and he is looking towards a loan signing to keep momentum going.
"We'll certainly sit down and look at that option because we can now see there's a freshness which would help the squad.
"I don't think there's too many midweek games left which is also good and we're currently on the end of a very hard season. These boys have done great to keep working away."
Another struggling side are set to provide further stiff resistance at the City Ground on Saturday as Wolves look to build on a 2-0 win at Millwall, their first three points under Dean Saunders.
"I was always confident we'd turn things round," said Saunders. "I know how to get a team to win - you've just got to keep believing what you're telling the players is right."