Don’t beat yourself up! Take it as a challenge and learning experience. Sure you didn’t ace it but maybe you’ll have a no fucks given attitude for your next one. I find when I don’t care if I stutter is when I don’t stutter. Or if I slip up I just mention I have a speech impediment and usually that relaxes me to the point where I’m not nervous about talking. You need to focus on changing your mind frame and your outlook. Again I’m sorry this happen to you and I can just imagine the feeling of worthlessness but it’s not the end you’ll have plenty more opportunities. Keep your head up!