I don't have anything particularly helpful to add except I understand how you feel - the anxiety before going into work, the temporary relief of missing it, and then further anxiety due to potential repercussions of missing it; I feel exactly the same though I am merely studying in health care, and working just a casual job. I cannot offer any other advice other than to try and remind yourself that once you are there, everything runs smoothly. Maybe your doctor can help you find ways of combating the anxiety.