Roberto Mancini's side kicked off the new campaign full of confidence after winning the club's first English title for 44 years in dramatic fashion on the final day of last season.
But that optimism is beginning to fade after a disappointing run of just three wins from their first nine games in all competitions.
England international Hart, who underlined his reputation as one of the best goalkeepers in the world with a superb display in Wednesday's 1-1 draw against Borussia Dortmund, believes his team-mates are finding it hard to deal with the increased expectations that come with being Premier League champions and also the extra desire opponents now have to beat them.
They face another test of their mental strength on Saturday as Martin O'Neill's unbeaten Sunderland -- the only team to take a home point off City in the league last season -- visit Eastlands looking to spring a shock.
It is exactly the sort of fixture that City, who are already four points behind leaders Chelsea, must win if they are to retain the title and Hart said: "I don't know if it is a learning curve, we are in with the big boys now.
"There is no place to hide. There is no point learning if you are not going to win.
"We were pretty open but it allows us to play forward. Hopefully we can shore it up again and be the force that we are."
If City don't win this weekend they are in danger of falling way behind Chelsea, who are heavily favoured to beat struggling Norwich at Stamford Bridge.
Roberto Di Matteo's side may not have reached their full potential yet, but they have still been good enough to win five of their six league games.
While new signings Eden Hazard and Oscar have earned plenty of plaudits after their impressive early performances, it is Spanish midfielder Juan Mata who remains at the heart of their best moments.
Mata underlined his class with two goals in Tuesday's Champions League win over Nordsjaelland, but he feels both he and the team have plenty of room for improvement.
"Maybe this was one of the best weeks for me at Chelsea. I'm feeling very well but I still want to do better," Mata said.
"Last year we won the Champions League and FA Cup. Now I think we can do it better as well."
Manchester United, four points behind Chelsea in third place, travel to Newcastle on Sunday as Sir Alex Ferguson's side look to bounce back after last weekend's 3-2 home defeat against Tottenham.
United's inconsistent start to the season -- they have already lost twice in the league -- and the goals of new striker Robin van Persie have made Japan midfielder Shinji Kagawa something of of a forgotten man since his pre-season move from Dortmund.
But United team-mate Tom Cleverley revealed Kagawa has made a big impact on the squad.
"We all know what Robin van Persie is about, but I've really enjoyed playing with Shinji," Cleverley said.
"He's really on my wavelength when it comes to playing one and two-touch football."
Arsenal will also be in the spotlight on Saturday as the Gunners, who dropped to eighth after their first defeat of the season against Chelsea last weekend, travel to in-form London rivals West Ham.
However, Gunners left-back Kieran Gibbs is convinced his side's midweek win over Olympiakos in the Champions League proved there is no hangover from the Chelsea setback.
"It was important for the players especially. We wanted to shake off a few doubts in our mind that we had. We certainly did that and we're back on the front foot now," Gibbs said.
"This is what we have to keep doing and keep pushing on from here."
Fixtures (1400GMT unless stated):
Saturday
Chelsea v Norwich, Man City v Sunderland (1145GMT), Swansea v Reading, West Brom v QPR, West Ham v Arsenal (1630GMT), Wigan v Everton
Sunday
Liverpool v Stoke, Newcastle v Man Utd (1500GMT), Southampton v Fulham (1230GMT), Tottenham v Aston Villa
Source: AFP
Source: AFP