You should really give yourself a break. Especially if you worked really hard the past two years (or however GCSE works in England :P ). I have only 3 exams left and halfway through I started to panic less as it all got very monotone, to the point where even when I knew the answer I was annoyed as they made me write it down (this will happen to you too probably - especially if you take many essay-based subjects like me, haha).
To be honest I might not get the amazing grades I was predicted, but I have to say I am happy with what I wrote so far - I know I tried my best and if the results don't totally reflect that, I won't mind.
Life is a bit more than these exams, and it sounds ironic since I'm the one saying it, but trust me, a day or two of free time will do sooo much good.