Make a mistake at your job or internship? Who hasn't? The question is what can you do to recover quickly.
Don't Beat Yourself Up
Just as the first line said, who hasn't made some sort of mistake at work. We're all human. Your boss is too. So don't spend too much time worrying about how bad you looked or wishing you could turn back time. You can't. Drewlling on it will just make you more likely to make another mistake and everyone else at the office has probably moved on by now. Learn from it, and move on.
Accept and Address It
If you made a bigger mistake that isn't so easy to just walk away from, then take some time to think through things and show your boss you are actively working to learn and improve. If you feel it is appropriate, set up a meeting to talk about or think about some other way to smooth things over in a professional manner while showing you are committed to growing and learning. Apologizing and accepting the consequences while showing that you are sorry is a much better way to handle it than trying to deflect blame.