Just copying out of the book and doing homework isn't really enough to get an A. In Chemistry, you need to know and understand all your basic stuff (mole calculations, common formulae, reactions and their conditions, all those types of things) and be able to apply them to the questions you're asked.
Unfortunately there's no quick fix for panicking in exams and making mistakes that you shouldn't have. Try to stay calm (not easy, but pretty important), and work on your Chemistry - if you feel more confident in it, you're less likely to panic (although don't neglect any of your other subjects).
Go through your mocks/completed past papers and find out where you're dropping marks - do you just need to spend more time learning things, are there problems in your understanding, is it exam technique, are you running out of time, etc. If there are understanding problems, go fix them as soon as possible! Talk to your teacher, and if they can't help there's always here. Also check that your notes don't have any gaps.
Find a more effective revision technique than just copying out. Flashcards can be pretty good for chemistry (you can write words/equations or names of reactions on one side and the definition/reaction and conditions on the other and use them to test yourself). Make posters or something of mechanisms you need to know, cover them up and see if you can write it out from memory. Try again the next day without looking again and see if you can still remember it - you have to keep going over things all the way through, not just looking at them once. Stick post its/posters of things you can't get to stick in a place where you'll see them every day.
Exam papers are a really useful revision resource, but there are a limited number of them. Look at how many you have left so you don't run out too soon. If you have access to the OCR Heinemann textbook this has loads of exam questions in as well that are really useful for practice.
You'll notice some questions come up very frequently on papers in almost exactly the same way - study the markschemes and make sure you understand what you need to write to get you the marks, as they'll often want to see particular words for questions that need you to write paragraphs (eg. the correct definitions for molar mass).
Good luck