After a fifth-place finish in the Premier League last season, Newcastle have struggled so far in this campaign and sit just two points off the relegation zone, having won just two of their last 15 matches in all competitions.
Krul, who gained significant plaudits for his impressive performances last season, believes that his form has slipped as his side have faltered.
"I can't be happy with my performances and it hurts me a lot," Krul said.
"I know the manager has shown trust in me, which is very important, but he gave me the chance here so I hope I can repay him.
"The last two games we've played fantastic stuff, but we've conceded too many goals.
"When you're putting a massive shift in and you've got zero points again, it hurts."
Krul, who has four caps for the Netherlands, has kept just two clean sheets in his 20 games this season, in comparison with 15 in 42 games in 2011/12.