The biggest challenge that students have with English is that they don't know what needs to be done to do well. I had this same problem, I would write an essay that I thought was good and got a mediocre grade, and submitted what I thought was an average essay and gotten a good grade that I was surprised with. So what are examiners and markers looking for?
ANSWERING THE PROMPT
Some people suggest memorising a few essays with different themes and then regurgitating the essay that is most similar to the prompt. I would advise against this, because each question has its different nuances and you must be sure that you are contextualising your knowledge of the text to the particular prompt that is put forward. Instead of memorising essays, memorise themes and ideas and then tailor it appropriately. Ultimately, examiners are assessing how well you synthesise symbols, characters, themes and values to answer a
prompt.
STRUCTURE
In order to answer the prompt well having a clean structure is necessary to helping the examiner follow your train of thought. Having a good contention or main point in your introduction is the place to start. Every other part of your essay flows out of your contention and that's why it's important to get it right. Furthermore, your topic sentences are subsets of your broader contention and contribute to your main idea. To make sure your topic sentence are effective and nuanced make sure it has both a cause and effect. For example, "the text highlights that past experiences (cause) result in the shaping of one’s worldview (effect)."
ANALYSIS
Analysis is all about zooming in at details in the text and then zooming out by showing how they relate to bigger values and themes. In each of the three essays analysis is crucial. To analyse well, look for subtle literary techniques that no one else picks up on and unpack their meaning and significance. Especially if you're doing a Text Response or Comparative, examiners love it when you look for symbols and motifs and link them with broader views and values.
Doing well in English is a challenge that takes time. It's not something that you can cram, but is a craft that develops over time. Ultimately, it is worth doing well in because it's going to be part of your top 4 whether you like it or not!