Mark Robins says his side need to step up in the final five games of the season after his City side were beaten 2-1 at Meadow Lane by Notts County.
Jordan Ponticelli scored for the Sky Blues but a second-half brace from Jonathan Forte was enough to send City home empty-handed.
"I am disappointed," said Robins.
"I am saying the same things week in week out. We should not be giving them goals from set-plays. We were masters of our own downfall.
"We worked so hard to get back in it having had the best of the game's play.
"The opening goal put us on the back foot and I thought that we dealt with that okay. We were still pressing. We were committing people forward.
"I don't think there was anything between the two teams. This has been a notoriously difficult place to come this season, but once we have the equaliser it only looked as if it was going to go one way.
"Then a stupid bit of play where we should have seen the ball out. Liam Kelly has tried to clear the ball and it has hit Jack Grimmer and everything was open on the inside for their man - but then it comes off a part of Jonathan Forte's anatomy and bobbled in over the line. It was poor."
Despite the result, the Sky Blues remain inside the play-off places with five games left to play.
"For me today is a disappointment because we are not ready, and we do not look like we are ready.
"I believe in my players, but we make the same errors every game. We have five games to sort it out because there is a big summer coming, regardless of what happens.
"The players have got to step up and the last five games have got to be cup-finals. Today was one of those games and we could have gone above them, but we have lost ground.
"We are still in the play-off but there are sides who can go above us on Tuesday."
He added: "The players are so disappointed too to lose in front of that away following. I have never seen this stadium so full.
"The fans are unbelievable, and the players know that. That is the hard part for us because we have done well to a point, and then not been able to see that through."