Panama looked the better side in the first half and had plenty of opportunities throughout the match but were unable to find the net before Caleb Yirenkyi knocked home the winner for Ghana in the 95th minute.
"The result is painful, but that's because we deserved better," Christiansen said. "We were excellent in the first half - we controlled Ghana."
"It is a cruel way to lose a game," he added. "But we can't just keep dwelling on this error forever."
Panama are still searching for their first-ever World Cup win and now face an uphill battle to get out of a group that also includes heavyweights England and Croatia.
"Here we have two games against very challenging opponents," Christiansen said. "We know the quality they have, but we will do our very best."
Panama's next match is on Tuesday against Croatia, who will also be looking to bounce back after a 4-2 loss to England earlier on Wednesday.
Christiansen praised Croatia as an "excellent team" but said his side will bring their top game and not be timid.
"The way we'll do that is by showing that Panama is a very difficult team to beat, and we are going to fight to the very end," Christiansen said.
