Stressed with new job

Work is very stressful at the moment. I feel like someone has handed me a bowl of cooking ingredients, partly mixed and said right off you go and finish the meal. I don't really know how the ingredients have been assembled, where we are in the cooking or mixing stage or even what the meal is supposed to look or taste like at the end! So last week when the woman from Together for Children arrived at first S and then the next day H I was seriously on catch up. There I sat with the lead bodies with all the feelings of your worst exam coming back. Friday night I felt a complete failure and have seriously wondered if I am up to this job.
I am determined not to give up and keep telling myself that I can do it but its reminded me of a long time ago when working for the library service and not really feeling able to cope with that managerial job then. I sort of gave it up and have regretted it as I know now I could do that job.
I also tell myself I am not stupid and if others can learn and do this job then so can I! To give up would damage my self confidence enormously and I am determined to find success.
I've just got to read more, work harder and stay calm and all will be well.
I have not been sleeping well lately. The last couple of nights I have had terrible wind or indegestion problems which I have put down to food or drink but whatever the cause I wake in the mornings feeling terribly tired and just wanting to go back to sleep.