You can do it, they know that you can do the job, that's why they gave it to you.
Relax, if you are still worrying after a bit, then promise yourself that with your first pay packet to get your hair done, if the next month you're still worrying get your nails done, for every time you worry about your work promise to treat yourself to something that makes you feel good about yourself on payday as a reward for coping and sticking at it.
You'll be fine, we're all rooting for you.