Creative work is hard. The act of creating something new from nothing requires extraordinary effort and perseverance. Often our work doesn't match the idea in our head. It's tempting to push responsibility for this onto someone or something else. "The client had ridiculous demands" or "I didn't have enough time to do research". This is counter productive. If none of the failings are on your plate, then there is nothing to improve upon.
Instead turn the question around. "I had a really short time to do this project, so what can I do to adapt my process to be more efficient?" or "I could have paid more attention to what the client really meant with her comments". Stand by your work, but acknowledge it's shortcomings in a productive way.
Every setback or failure is really a chance to improve, if only you know how to frame it.