The group's latest album, Love.Live.Life, reached number seven and spawned three top 20 hits.
In an interview with The Observer, Dappy admitted: "I get depressed. I've gone to dark places for days when I've seen our music videos have dropped off the TV airplay charts. Or I read a rude comment about us on YouTube. It fills me with self-doubt."
The rapper also told the paper that he wanted N-Dubz to achieve greater record sales than they have previously.
"Going triple platinum isn't good enough," he said. "I'd like to go quadruple platinum.
"We've sold a hell of a lot of albums over the past few years, but it's about exceeding what you've achieved. It's easy to get there - but holding on to the success is much harder."