There is no need to panic, and researching articles about how bad driving it is to drive on motorways is not helpful to you.
But I do know the feeling, even though my situation is vastly different. I moved from Australia to UK for work and was given a company car on my first day. After being in the country three days, I was expected to drive a few hours on M25//M1 to Wolverhampton. Cue - major panic.
The roads are so much narrower than Oz, and the car I had been given was huge compared to what I drove in Australia. I panicked in the car park on the first day of work because the car park was clearly only designed for minis or smart cars and not massive saloons. It took me 10 minutes to get into my designated space.
Much like you I had worked myself into a right state! I hadn't experienced variable speed signs (I found them more annoying than confusing). Add to that the car I had was so powerful compared to my previous car that I kept finding myself driving 80 or 90mph and was convinced I would get speeding fines. Every time a lorry was next to me I panicked (though the ones in Oz are far more terrifying).
Well after a couple of weeks of anxiety every time I drove, things got better. It was just lack of experience of the car, of the tight spaces. Mind you it was a factor in deciding to rent a place WALKING distance to work!
Enough about me, the fact is you said you drove all fine.
It was just your mental state. One thing I did to help me cope with the fear was to repeat to myself almost as a mantra
"hundreds of thousands of people are driving along the motorway with me, with no problems, there is no reason to panic, this is a normal activity and I am a normal person doing a normal activity". It calmed me down.
It might make me look like a loony, but it worked for me.
Also, so what if you get one speeding fine. Its not the end of the world. Don't let fear hold you back.
So get out there and drive and you will see it will become better with experience (honestly it does!). The worst thing to do is to try and find other anxiety sufferers and indulge yourself in needless melodrama!