Roberto Mancini's side have already been knocked out of the Champions League and pressure is mounting on the club's Italian manager, despite him only signing a new five-year contract last summer.
That has prompted an admission from goalkeeper Hart, whose poor positioning allowed former City winger Adam Johnson to score Sunderland's winner at the Stadium of Light with a long-range shot, that they have found it harder to defend their title than they anticipated.
"We have dominated a lot of games, but people play differently against us now," explained Hart. "We have to adjust to that and hopefully the second half of the season will be better for us.
"Teams are definitely playing differently when they face us, but that comes with being the champions. It's tough when you play good football and people defend against you, but that's what it's like at the top, it's tough.
"No one else needs to know what gets said [in the dressing room], it's down to us. To be leading is difficult, but chasing is even harder. Hopefully we can do what we did last year."
City are struggling to replicate the form they showed when beating United to the title on the final day of last season and with half the season gone, United are threatening to burst clear in the title race. More worrying still is that City's defeat at Sunderland might have been even more emphatic.
"They did what they had to do to win and get the points," added Hart. "We knew all along it was going to be difficult there, it always is. We started really well and dominated the early stages, but that's the way football is, you have to score when you're on top and we didn't this time. If we had scored in that period it could have been a different story.
"It's how football goes sometimes. It's not that we didn't try, didn't give 100 per cent, or haven't shown qualities. It just wasn't there and we need to turn that around quickly. We weren't lacking in effort. We gave it everything, but couldn't break through a Sunderland wall."