What an essay is for
An academic essay makes a claim and defends it. That sounds obvious until you look at how most essays are actually planned, which is by topic. A writer given the topic of social media and adolescent wellbeing will collect material about social media and adolescent wellbeing, arrange it in a sensible order, and produce something that reads like a report. It will be accurate and it will not argue anything.
The difference is a position. Not what do I know about this, but what do I think is true about this that a reasonable person might dispute. Once you have that, the structure of the essay is the structure of your reasons.
Planning
Plan in claims, not in headings. Write your position in one sentence, then write the two to four reasons that make it true. Those reasons are your body sections. If a reason needs more than one paragraph, it needs sub reasons, and those are your paragraphs.
This is faster than outlining by topic and it exposes weak arguments early. A reason you cannot state in a sentence is a reason you cannot defend in a paragraph.
Introductions
An introduction has three jobs: establish the question, indicate why it is worth asking, and state the position the essay will defend. It does not need a general opening about how throughout history humans have always. Cut that sentence. It is always the first thing a marker skims past.
Length follows from the jobs. Ten per cent of the essay is a reasonable expectation, but if yours is running long it is usually because it has started making the argument instead of announcing it.
Body paragraphs
A body paragraph should be able to answer four questions: what is the claim, what is the evidence, what does the evidence show, and how does this connect to the argument. If you cannot find the claim in the first two sentences, the paragraph probably has two claims in it and needs splitting.
The most common weakness is the paragraph that ends on a quotation. Evidence does not speak for itself. The sentence after the evidence is where your analysis lives, and it is the sentence most often missing.
Conclusions
A conclusion should not summarise. It should land. Say what follows from the argument you have made: what it means, what it changes, what it leaves unresolved. Restating the essay in miniature wastes the one place the reader is paying attention again.
Revision
Revise in passes, one problem at a time. First pass, structure: does each section earn its place, and are they in the right order. Second pass, paragraphs: does each one make one point with evidence and analysis. Third pass, sentences. Trying to fix all three at once is why revision feels endless.
The most useful single technique is the reverse outline. After drafting, write one sentence describing what each paragraph does. Read that list on its own. If it does not read as a coherent argument, the essay does not either, and no amount of sentence polishing will fix it.