Gwyneth Paltrow and Chris Martin on October 25, 2012 in Santa Monica, CA. Credit: AKM-GSI

Gwyneth Paltrow is honest when it comes to discussing her marriage -- it's hard work!

The Iron Man 3 actress celebrates her ninth wedding anniversary with husband Chris Martin on Wednesday, Dec. 5. In a new interview with InStyle UK, Paltrow, 40, admitted that she and the Coldplay frontman, 35, have faced challenges in their relationship mostly due to their busy careers.

What's the hardest part about their marriage?

"Probably our artistic temperaments together," Paltrow said. "Artists are sensitive; there are ups and downs, mood-wise. Musicians need a certain gravitas and focus in order to write."

The actress is set to reprise her role as Pepper Potts in the upcoming Iron Man 3, and Martin has been touring with Coldplay for their Mylo Xyloto album. Paltrow explained, "The temperament that goes with someone who is creative can be challenging." 

In addition to their busy careers, they are also parents to daughter Apple, 8, and son Moses, 6. But Paltrow wishes they could have more children!

"My brain says, 'Oh, I think I'm done, the kids are big now and I don't want to go back to changing diapers.' But a part of me would love to have another."

"Of course," she added, "I'm old now!'