I struggled a lot with essay writing, it's still not perfect now, but I've improved quite a bit so I'll try give you some tips that worked for me:
1. Look at the AO's and break them down into basic things that are easy to remember; for example, I know to include; language, form, structure, context, link to other parts/events of the text, and part of the Ao's are structuring the argument, including different interpretations, and personal opinions.
2. I don't know what your teachers are like, but if you are able to, ask them for personalized detailed feedback, and ask, "What can I add and change in my essays to gain more marks."
3. Find essays that are from the same exam board as you, (student exemplar essays), and try to find ones that have low marks, and ones that have higher marks. Read the low mark essays, highlight/underline things that they include that are in the AO's, and make a note of how the paragraphs are structured (such as point, evidence, analysis/in depth analysis, context etc), then do the same for the higher mark ones and compare what they do differently. It would be even more helpful if you could ask your classmates to email their higher mark essays to you, and you can see what they include that you aren't, and try to use some of the techniques you see being used that you think are useful.
4. In terms of general essay structure, here's a couple of tips; Some of it you probably already know from gcse, but hopefully some of it will help. Also keep in mind this is based on the AO's for my examboard, so some things might not apply for yours.
Planning is key; this makes sure you don't ramble and stick tightly to the question. I'd say, read the question, highlight the key words, come up with three ideas that you're going to discuss in your essay and jot down examples/quotes you could use to prove those points.
Introduction: Outline the argument you'll be talking about, for example if it was about the theme of deception in Much Ado,
"Deception is a central part of the comedy “Much Ado About Nothing”, acting as a primary source of drama throughout the play, especially as it highlights the complex themes of deception versus reality." then I would go on to briefly mention the three/
ideas I came up with, so
"Shakespeare presents deception in different ways, using the romantic deceptions between Beatrice and Benedick to establish a lighthearted atmosphere which contrasts with the conflict and tensions that arise as a result of the more serious deception.... etc". (I've only included two ideas in the example btw) Also, if I mention an idea that links nicely to context, I'll introduce that bit of context into the introduction too.
Another thing to keep in mind when writing, is
avoid being vague, so in the examples I gave above, by mentioning the characters names, this demonstrates one of my AO's (knowledge), and by specifying what kind of atmosphere, and using adjectives I'm being more evaluative and analytical. Also, sometimes when you make points that aren't specific, you raise more questions which is what you want to avoid.
Main paragraphs; After the introduction, take it one paragraph at a time to tackle each idea you came up with.
Start with a
topic sentence (briefly introduce the idea you'll be talking about within the paragraph)
Then try to bring in a
quotation that demonstrates the idea, then
explain the quotations general analysis, then if there are certain things (sentence structure, punctuation, a strange word, adjective, etc) you can
zoom into and analyse further, do so.
Then maybe provide an
alternative interpretation or another quote that supports or contradicts the point, allowing you talk about it more.
And through out doing this, try to throw in something about
context that links to your point (the period it was written in, how people viewed it, something to do with the author etc), this only needs to be done a couple times in the whole essay, so unless necessary don't do it more than once per paragraph. Also add in somethings to do with the effect on the reader (empathize, ridicule, question, consider etc)
Lastly;
Always, always LINK BACK TO THE QUESTION. To make sure your answering the question, and to make sure the examiner knows your answering it, always refer to the question, you can do this by rephrasing the question at the start of a paragaph, or by using the key words within the question throughout your paragraphs as you make your points.
Hope that helps
if you have any questions let me know.