First of all, don't stress about past exams. They're done and dusted, and you can't change the past. I know that there have been some important exams for me this spring where I've finished and then spent the next few days super stressed about whether I got a question wrong or not. You have another opportunity to do yourself proud, so just put all your focus on that. If you're sitting on a high B, all you need to a good paper to push you up to an A. My biggest tip would be to treat the last exam like a practice or mock, and simply take in what you learnt from it. Make sure you don't repeat any of your mistakes from it. Next time you'll be aware that you might be rushed for time, and will know how to spread it effectively. Then, do lots and lots of preparation. Make sure epyounknow the intent, but also are familiar with the style of questions and layout of the paper. I don't know how long your topic is, but make a list of all possible essay themes, and then write an essay on them/plan out what you'd say - preferably under 'exam conditions'. Also, have a look at the examiners report for your exam. It might not apply too much, but you'll be able to see where others may have gone wrong.
Hope this helps, and your next exam goes well!