Sunandmoon, can you try and list the reasons why they offered you the job: perhaps it is because you are organised, calm, compassionate, mature, good at problem solving, good with people, intelligent.... lots of reasons. None of these will have changed, you are still that person, you probably just haven't found your feet yet, so haven't been able to apply your skills and experience.
Rather than finding 'everything' in your job stressful, can you try to identify the key things that you feel uncomfortable with, you might find it is not as many as you first thought. Then you can think about how to get help for each specific area, and set yourself a time-frame. For example, if new medical terminology was one of the problems then set yourself a challenge of learning 10 new terms each week for 6 weeks, then review to see whether it is still such a big worry.
Also think about the parts of your job (however small) that are OK really. And remind yourself how you are using your skills in those areas.
I have heard change being described as similar to a trapeze artist changing from swing to swing... They set up the momentum in the swings, and are practiced in making the leap, but there is a moment in the middle where they are in mid-air, not actually holding on to anything solid on either side. That may be where you are just now. Not surprising it feels scary, but the worst bit doesn't last that long - reach for the positive things on the other side. I am sure you will find them.