Substitute Ryan Jarvis gave Orient all three points with a last-gasp strike just as the game looked to be heading for a 1-1 draw thanks to a Gabriel Zakuani own goal and Posh substitute Nathaniel Mendez-Laing`s headed leveller.
Ferguson said: "To lose in the manner we did is very upsetting. I have to look at it and say that as a group I really think we could be fitter. We conceded a late, late goal which is down to a lack of consideration and we've also had three players go down with cramp so that's something I've got to look at.
"We've got a good mentality and don't deliberately make mistakes and we don't deliberately lose games and I have such belief and confidence in my squad that I don't think we should be losing any games."
However, Ferguson added: "It's a blow to our automatic promotion hopes. I've not given up although I would have to say it's unrealistic to think we will finish second.
"But now we've got to try and make sure we finish the season with three wins and go into the play-offs full of confidence. Despite the result, I'm still very confident that through the play-offs we will get promoted."
Source: PA
Source: PA