It is
inevitable that you'll find mistakes so it is absolutely necessary to check, check check.
I did my Edexcel paper one last week and if I didn't check through it, my marks would've easily dropped from the mid 80s to the low 70s without a shadow of a doubt.
You also want to revise stuff you really don't know. Don't bother revising completing the square if you know it, revise something you don't know. You'll know you don't know it because you can't do it in a past paper question.
In the exam, you want to read the question so carefully. What is it asking you to do? It's so easy to slip up and do the wrong thing, trust me. Underline key information and key command words in the question. Is it asking you to rearrange or solve??
Ask yourself, why did you get 67 in that paper and why not 100? Ask yourself where the 33 marks went and how you'll tackle this in paper 2. It's all a learning experience.
Finally, relax... if you panic in a maths exam things go wrong and it's a domino effect. You can't do one question so you skip it. You can't do that question either so you skip it. Relax... I also skipped some questions and forgot about them. Then when my brain suddenly clicked I'll immediately go back to it and say to myself "How easy was that?!" thus I'll gain more confidence and do lots better.
I start from the back of the paper so I don't get cocky with the foundation parts and make a stupid mistake I won't even bother to check because I am over confident.
NB: answer all questions and put something down. Even if you have no idea, do something within reason. Your working gets more marks than the correct answer!
Good luck!!!!