Jewlz, just wanted to add to the chorus -- you're not alone!!
Like you, I drove for years and years and years in the US, and while I was not a happy camper regarding the aggression in the traffic of my neck of the woods, technically I was a fully competent, confident and capable driver in full control of the vehicle and full command of the conditons around me and of my observations.
This perfectly confident driving went on for seventeen years, my only accident being when a 17 year boy hit me.
Jump to a couple of months ago, London, England. OH. MY. GOD. !!!!!
I'm an incompetent FOOL! I was driving like a KID all over again -- scared, not observing well, freaked out at sitting on the "wrong" side of the car, freaked out at driving on the "wrong" side of the road!!!
Everything was the wrong way around, mirror image, my "wrong" hand had to control the stick, I kept pawing the air with my left hand -- I was such a nervous kitten that I almost forgot to notice pedestrians, cyclists, other vehicles, road markings, street signs, speed limits. And I hated how narrow the streets are, how many parked vehicles obscure most of everything you have to do, how many pedestrians there are to deal with -- in my US city the streets were wide, and almost no pedestrians to distract or deal with.
Over here it was hell! And I felt angry and useless and humiliated because, for seventeen years, I had driven all day every day of my life, perfectly competently, without thinking about it. I was comfortable on the left hand side of the car and the right hand side of the road. Aside from a tendency to impatience and road rage and being fed up of the traffic, I was fine over there behind a wheel.
Granted I'm now driving in a city where even other people in the UK say "Oh God I could never attempt to drive in London."
But I'm actually starting to get used to it --- and you will too. I thought I never would or could, but its starting to come now, and I think I'm going to be okay.
So take heart and hang in there -- it may feel like you want to say goodbye to driving, I did too, but keep on trying.