City were beaten 2-1 at Villa Park, meaning City have now won once in their past 12 games.
Afterwards, Haaland said: "It's a couple of minutes since the game, so not a lot (has been said). But of course we are disappointed, it's not good enough. It's not good enough from me.
"They're (Aston Villa) good players, it's difficult to come here but we're Man City we should be out there. We have to continue, we have to believe and we have to keep working hard.
"We have to continue. First I'm looking at myself, I haven't been doing things good enough, I haven't been scoring my chances. I have to do better, I haven't been good enough."
On confidence levels, he continued: "Of course, not the best. We know how important confidence is and you can see that it affects every human being. That is how it is, we have to continue and stay positive even though it is difficult."
On manager Pep Guardiola, Haaland also said: "He won the Premier League six times in seven years, so we will never forget that. He will find the solutions. He has been doing that every single year. We still believe in him, we have to work harder than ever right now.
"It's the small details (what Guardiola told him after the final whistle), I don't remember right now. I'm thinking we just lost a game, another one. I don't remember but it was probably some small instructions to do things better."