Nerves are what they are. They are hard to control. But an interviewer usually only has so much time with you before they need to move on. Sometimes the best way to make good use of that time is to take a deep breath, and answer their questions slowly, and clearly. Use the time to your benefit.
I had another interview with a young woman who was very thoughtful with each question. After each question she would pause for a solid two seconds, clearly composing her response. This allowed her to clearly, and confidently answer my questions, and she rocked that interview even though we had to stop a few questions short of the list. She made excellent use of our time together.
I think the biggest part of this is the candidate's mindset. I can't help but think that the first candidate was a little less sure of herself than the second. I think everyone has something to bring to the company, the only question is "will the hiring manager see it?" If this company doesn't see how they need you, then your job is to relate to them what you bring to the table. Help the hiring manager see why you are the solution to their problem. In order to do this you have to understand your strengths, and be prepared to "sell" them. If you don't see a clear link between your capabilities and the company you are interviewing with, then you shouldn't put in for that position.