Although many students (and quite a few teachers!) might disagree, analysing historical texts is one of the great joys of studying A-Level English Language. Admittedly, picking your way through the archaic lexis, grammar and orthography isn't always easy - especially with texts written before the 18th century - but once you're in, you enter a different world.
The trick to analysing such texts well is to avoid saying boring stuff about certain words not existing any more or that they don't have any grammar. Words don't vanish, they just 'fall from mainstream usage' and the only texts without grammar are ones without any words! You need to get a feel for the texts as living things. They were written by real people with real feelings about the subjects they were discussing. More significantly, the words on the page were as relevant to them at that point in history as the WhatsApp or Snapchat you sent five minutes ago. Importantly, language should not be regarded as more or less important - or superior or inferior - based on when it was produced. Simply, it is what it is.
I especially enjoy diary entries and letters from the past because they were intensely personal. They had no intended audience (at least, not a mass audience) and so they were written frankly, honestly and without affectation. They weren't trying (in most cases) to influence the reader. That said, a letter from a monarch to a politician would probably have contained some ulterior motive.
The other things that trip students up are the attitudes expressed. It's all too easy to write an essay dismissing a text as sexist, racist, superstitious or ignorant in some way, but this can be a misrepresentation. These texts shouldn't be analysed in terms of social attitudes today or how we think today; they need to be considered as a reflection of the time at which they were composed. Back then, attitudes based on gender, sexual orientation, race, social status, religion, the supernatural and so on were different. Centuries ago, there were no such things as political correctness, equality awareness or cultural sensitivity (as we understand them today) operating in the mainstream arena. Often, people were simply writing what was generally held to be true or, at least, acceptable at the time. There are exceptions of course. Nobody could read the vile rantings of a madman like Hitler and dismiss his words as culturally uninformed just because they written almost a century ago. There are some things that common sense alone leads us to recognise plainly as being only evil and wrong.
More often than not, you can discern much about the text you are analysing by reading the contextual information provided by the examiner and looking at the year of publication or origin. Sometimes you will see that a text was written in or around a significant year. If so, ask yourself if there are any parallels that can be drawn between the attitudes expressed and the point in history that it was written. Above all, just remember that the attitudes and ideas expressed may only seem ignorant or offensive when weighed against today's knowledge and social values. As ever, historical knowledge is important and context is everything.
The one piece of advice I would leave you with is this: don't be scared of an old text. Trust your revision and remember that the examiner won't ever ask you to analyse something inaccessible.