You're not alone, before my driving lessons (though only towards the end, I was never nervous before my first 10-20 or so) I would feel absolutely sick. Same as you, I'd be fine once I got in the car, but if it was in the morning I'd never be able to eat anything and I felt physically sick. I also never understood why, and still don't today, but I passed and it's all good, so I think it's just nerves at the thought that something could go wrong.
You probably know you're a good driver, maybe you're nervous of messing up a little as you know you can do it. Then in the car there's no time to be nervous as you're concentrating so much. I wish I had some tips, but I never figured out how to get round it myself, other than distracting myself from the thought of it. Try watching some TV or doing something you enjoy to take your mind off it (but don't forget about it altogether and miss it!), and trust yourself as a driver.