'There is no such thing as a failed interview.'
Now let that sink in for a moment. Read it back to yourself. Read it slower.
You cannot fail, you can only produce results. Rather than judging some result as a failure, ask yourself, “What have I learned about the things that don’t work?”, and "What can I do with these results?"
Henry Ford's first two automobile companies produced bad results. He could have packed it in, but he didn't. He learned from any mistakes he made and continued to improve and eventually got the results he was looking for.
Steve Jobs made an incredible amount of mistakes in a 'trial and error' approach before he eventually got the results he was aiming for and made Apple the brand it is today.
Sachin Ramesh Tendulkar, the acclaimed 'god of Cricket' had once stated, "I am not the god of Cricket. I make mistakes, God doesn't."
A famous Bruce Lee quote was, "Failure is simply the opportunity to begin again, this time more intelligently." What we take from this is to learn from your experiences and build on them in the future.
So you see, everybody makes mistakes. Your job is to learn from them, especially when it comes to the interview process.
On average, the ratio of interviews to job starts are quite high. In general terms, that means that candidates have a higher number of interviews under their belt than jobs started.
If you're a candidate, you need to keep in mind that you won't get every single job you apply for. Nobody does, and that's okay! After all, what would be the point? If you got the first job, and you took it, you wouldn't have any more interviews, right?
What you do need to remember is if (and when) the day comes around when you get told you were not successful, then you need to take positives AND NEGATIVES from your interview experience.
The negatives you remember are just as important - if not more important than - your positives. These are what you need to work on and improve on.