Joss Whedon felt like a "miserable failure" after directing 'The Avengers: Age of Ultron'.
The 51-year-old director was left "exhausted" by the filming process on the superhero movie after clashing with Marvel Studios, who he has since quit, and heavily critiquing his own performance as the filmmaker.
He said: "The fact that Marvel gave me that opportunity is so bonkers, and so beautiful, and the fact that I come off of it feeling like a miserable failure, is also bonkers, but not in a cute way.
"I was so beaten down by the process. Some of that was conflicting with Marvel, which is inevitable. A lot of it was about my own work, and I was also exhausted."
Joss has admitted one of his biggest regrets he has about the motion picture was that he downplayed the movie ahead of its April 2015 release, but he now realises that didn't do the project justice.
Speaking at the Tribeca Film Festival, he added: "I think that did a disservice to the movie and the studio and to myself.
"It was not the right way to be, because I am very proud of it.
"The things about it that are wrong frustrate me enormously, and I had probably more of those than I had on the other movies I made, but I also got to make, for the second time, an absurdly personal movie that talked about how I felt about humanity, and what it means, in very esoteric and bizarre ways, for hundreds of billions of dollars."
In January, Joss confirmed he had made a "clean break" from Marvel, but insisted they had parted ways amicably.
He said: "You know I was their sort of conciliary for a while. We do not discuss our thing ... but, I sort of had my finger in all of the films in the second phase, but then I just had to concentrate only on 'Ultron', and sort of knew when it was done I was just going to stop ... So I made a completely clean break - not because we had a falling out - just because I was like, 'I can't...' If I was still there going, 'Well, here are my thoughts on this film,' I'd be there every day.
"I wouldn't do anything else because there are a lot of films, and it is a lot of fun. It's very seductive ..."