Before my GCSEs, I spoke to a person who is two years above me who said that students tend to reflect too much over their mistakes and generally underestimate their performance. I was also worried from about a week before the results day and had a habit of guessing where I lost marks. It turned out that I did in fact underestimate my GCSE performance by five A*s.
I didn't really care about AS Level results until about a week before the day as the Olympics were there to distract me. Though I did panic on the day as I overslept which left me getting up at 9:40 am. Turned out I actually did fine that time too.