I just passed my test but failed the Part 1 the first attempt. Like you the Part 1 had me much more nervous that the Part 2. I really stuffed up the first time I practiced for the Part 1 which was why it had me nervous. I failed the first time on slow riding (when it was the swerve that actually made me more nervous) and then followed my instructor all the way home filtering through traffic in town - that was annoying.
My advice would be,
Take your time, there is no rush. I found it really helpful to just take a second to run through in my head what I needed to do next before i did it (like for slow riding, don't touch the front brake, use the back, plenty of revs etc.)
For the slow riding, look where you are going/at a point at the far end of the playground not at the examiner. I did the figure of 8 nearly 3 times cos I was concentrating on where is was going and not at the examiner who was waving me over.
Also for the figure of 8 you don't have to do it tight round the cones, go wide and use as much space as you want (you are allowed to go around the last yellow cone in the slalom row.
Also - if you do dab a foot down just carry on like it never happened. I put my foot down but he hadn't noticed it till I owned up after in his office (I'd failed already anyway so it made no difference).
Also you were right to carry on - my instructor had a guy learning about the same time as me who made a mistake near the end of his Part 2 and assumed he'd failed. He rode back to the test centre not really trying only to be told that even though he'd made a mistake his riding till that point had been good and the mistake would have been overlooked if he hadn't ridden the rest of the way like a tit.