My problem is expressing myself properly. I often stumble during sacs and repeat myself or/and ramble on. So some tips from some englishy people would be great! 
I had your exact same problem during this year, especially in my essays!
My advice to you would be to plan out your essays in advance for SACs, and make sure your plan includes a topic sentence specific to the paragraph you are writing, the evidence/quotes you use and how these link back to the original essay question. This helps you to keep your paragraphs on track as you know the meat of the paragraph - the topic, and your contention.
If you have time before a SAC, maybe write out your essay on the computer directly from your plan and see where you tend to ramble. Prior to a SAC, you have time to edit yourself, so do! Re-write where you ramble and that edit may stick in your head when it's actually writing time.
Also, commas and voice. If your sentence has to be long and rambly, put in some commas to make it easier to parse. Also, trying to write in active voice ("Australian English is an important facet of national identity" --> subject verbs object) over passive ("Use of Australian English provides an important marker of national identity" --> object is verbed by subject) helps to make your writing more concise.
And I rambled here. Apologies! Hope this helps
