You shouldn't doubt your capabilities with the job, your stutter has no correlation with your knowledge of the job at hand. You will have plenty more opportunities for interviews, don't beat yourself up. You know there is confidence in yourself and what you can do, you have to keep reminding yourself of it. A good trick a lot of people use is to try trick your brain into thinking and believing that your nervousness is actually excitement - try it for next time. There will be plenty of next times!