Some good advice above.
I failed my first test, same as you 2 majors, two things I'd never done before and never done since. The first I was waiting at the entrance to a busy roundabout and had been there some time, I started to panic thinking I was about to fail for hesitency so I took a gap not quite big enough

, the second one I'm too embarassed to mention

.
As for nerves, well sadly there's no cure, although I had a guy once who had a great attitude towards it. His philosophy was that he was going to channel his nerves into a positive, use them to keep him 'on his toes', as he said to me 'if I'm too relaxed I could end up with a sloppy drive', this is alot easier said than done but it worked for him.
Have a good breakfast on the day of your test, your stomach maybe in knots but eating helps concentration (Banana's are good I'm reliably told).
Do as many mock tests as you can, if possible with an instructor you don't know but with the same car, you'll never re-create exact test conditions but it's good practise.
Try not to over anylyse situations! When on lessons you see a situation, assess the options, decide on the best course of action and then do it, this takes a matter of seconds. Sometimes on test though people see the situation, assess the options, decide what they would normaly do, then think what would the examiner prefer you to do, then think but if I don't do this will the examiner think I don't know.......by then it's too late! You say in your post "I'm a good driver", if so trust your instincts and do what you normaly do in lessons, nothing more, nothing less.
Put your first test down to experience and try not to dwell on it. I can say from experience the second time round is much easier, and although nerves will still be there, you'll know now what to expect and hopefully how to deal with it.
Above all GOOD LUCK, keep us informed!!!