"We have had leads, United have had the lead -- it's to-ing and fro-ing at the moment," said Hart.
"We are a team of winners. It's hard and it hurts, but we have got to move on. It is hard to lead and it is hard to chase too.
"We have clawed points back before, we proved it at the end of last year. We have just got to believe."
Sunderland's record against City is remarkable -- they were the only side to leave the Etihad Stadium last season with a point -- and Mancini joked afterwards that they would not travel to Wearside during the 2013-14 campaign. In the short term, the Italian's focus will be on tomorrow's trip to Norwich, where his side won 6-1 in April and where a repeat of that result, if not the scoreline, would be hugely welcome.
Hart said: "The only way we can change it is by winning here (at Sunderland) next year -- but for now, we only have Norwich on our minds."
Norwich had gone 10 league games without defeat before losing 2-1 at West Brom on December 22, and they were unable to make amends on St Stephen's Day when Chelsea left Carrow Road with the three points courtesy of Juan Mata's first-half strike.
The Canaries could be forgiven for approaching the fixture with some trepidation after losing the corresponding match last season 6-1 on a day when Carlos Tevez hit a hat-trick and Sergio Aguero struck twice.
Johnson completed the scoring on April 14 and, by coincidence, he found the back of the next on Wednesday, although this time wearing the red and white of Sunderland against City.
Hart was unable to keep out his former team-mate's long-range 53rd-minute strike on a day when opposite number Simon Mignolet frustrated the visitors with a string of fine saves.
Hart said: "We dominated the game and lost, but we have got to move forward. Their goalkeeper had a special game, which he has done against us recently. He is a good keeper. We just couldn't quite get it in."