I've been studying Latin for years now, but I still struggle with the unseen prose paper. What can I do?

The key to translating Latin is reading through the text several times, without worrying about translating words immediately. If you are taking OCR exams, then, you're sure that the text will be by Livy, and this is helpful for two reasons. Firstly, providing your preparation on the author is sufficient, you can spot in the text the author's peculiar use of structures, syntax and phrases, as well as his exceptional use of grammar. Secondly, Livy is a historian, meaning that a solid ground in Roman history can help understand the context whenever the passage is about a historical event.After reading through the text, you should focus on syntax at first rather than on vocabulary, because often the former can influence the understanding of the latter, while the opposite hardly ever happens. In order to do that, spot and analyze the verbs and their tense, so that you can realize which kind of clause they are part of. Afterwards, identify the main and any secondary clauses, as related to the main, and draw a diagram if the sentence structure is complex. After this thorough process, the vocabulary should come easier to you.