Crawley striker Matt Tubbs settled the issue from the penalty spot five minutes from time after debutant Posh defender Jack Baldwin had handled Andy Drury's free-kick.
Ferguson is disappointed his side have not fared better against teams in the bottom half of the table.
He said: "We haven't taken enough out of games although we did come close right at the death when Conor Washington hit the post. That would have been a good equalizer.
"Crawley started really well, but it looked like a 0-0 before the penalty came.
"Baldwin did handle, but I felt the striker pushed him on to it. If a goal was to sum up our season that would be it.
"Baldwin came on as Ben Nugent was struggling with a hip injury and was struggling to run 10 minutes before.
"He will be a doubt for our game at Sheffield United on Tuesday.
"I don't like to make excuses, but we were too inconsistent in the final third and with our final touch.
"After watching Crawley at Swindon on Tuesday, I went with the same team as played against Stevenage and there were one or two disappointing performances."