After working on Marvel's blockbuster hit "Avengers: Age of Ultron" last year, everybody assumed that director Joss Whedon would simply continue working with the studio and create one hit movie after another.

However, working on the movie really took its toll on Whedon and he decided to quit working with Marvel after that. The director admitted to Cinema Blend that Marvel has been great to him, but he needed to let them go because his creative juices needed refuelling.

"I've gone off the reservation for a while. It was five years that I was working on either 'Avengers' or 'S.H.I.E.L.D.' That was an enormous gift they gave me; they handed me several hundreds of millions of dollars and said 'do what you do,' which is very rare and I was very lucky," he said.

"At the same time, it's important for me not just to have my own thing and do something smaller, but also to create a new challenge for myself because I will start to repeat myself," Whedon added.

Earlier, Whedon admitted to The LA Times' Hero Complex that working on "Avengers: Age of Ultron" really broke him.

"It's weird because the first one was very, very, very, hard. This one was much harder. It a little bit broke me," he said.

Whedon cannot fault Marvel execs and the entire cast and crew of the film because they were all "lovely" to work with. His biggest enemy, Whedon revealed, was himself.

Whenever he told himself, "I want to make something great," a voice inside his head would nag at him, "But is this a great idea?" After completing a scene, he would feel happy with what they have accomplished but then the nagging voice inside his head would say again, "I don't know Joss, could you have done better? You could have done better."

Whedon suffered from self-doubt, and because of this, the studio did too. Given the hype surrounding the movie, all of those working on it felt immeasurable pressure. "Everything's riding on this all of a sudden. And it became a problem in a way that nothing else has. And it was a hard movie to make on top of that. So being paralyzed by either indecision or the weight of responsibility? Not useful, don't have time for your paralysis, son, snap out of it. This was the hardest work I've ever done," he said.