I am so nervous about my first real job. Help?

I am on my third day as a trainee. I can't seem to set things straight for myself lately. Im so nervous that my eating and sleeping patterns are so affected. I'm so scared to disappoint the people around my first job because they seem to be very kind and have some high expectations about me. But once I tried doing the hands on work, I'm like a child who doesn't know what to do. I feel so stupid up to the point that I just wanted to quit. Please help me. I can't seem to manage myself anymore.