Coherency of your essay starts with looking at the bigger picture. This is why planning is so important! If you start planning your essays, you already know what ideas belong to which paragraph, and whether they all link together.
I think many students don’t realise that a lot of the hard work is actually done before writing an essay because this is where you:
* Breakdown your essay topic
* Brainstorm arguments/contention
* Come up with your contention and main arguments
* Ensure that your contention and main arguments link
* Ensure that your main arguments back up your contention
Once you know exactly what you’re doing, the writing part should come easier because the ideas are clearly outlined in your head (of course, it’s not just that simple since you still have to deal with expression, idea development etc. but say if you go into an 18th birthday speech without having any ideas of what you’re going to say, it’ll probably come out more jumbled than if you have an outline).
P.S - Honestly, don't stress about your study score. Just use this precious time to actually focus on your weaknesses and do the hard work to get there. The more people spend worrying about their ATAR, calculating their ATAR etc, the more time that's taken away from you actually studying and improving.