What do you actually have to do in the assessment? Like a speech, or a series of questions?
In all my speaking exams they've been different sorts of things.
Revision tips (woooo
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Try recording yourself saying it and then just listen to it whenever you can. This really helps because you memorise it without thinking about it. So, like, on a train/walking somewhere/before you sleep etc.
Get someone to listen to you, make sure you can't see a sheet with it all on, but they can, so they can stop you and correct you (or prompt you if you forget it) if you get any of it slightly wrong.
If there are some parts of it you're really struggling with, maybe use post-its and write little parts of it and stick them next to places you often go, e.g mirror/next to your bed etc. This is good because then you're constantly looking at the bits you're confused with, rather than a whole block of text which you might know say 80% of... Obviously you have to look at the actual one too, that's just a bit on the side.
You could also try learning it gradually, so like, a paragraph over a week or something (adjust to the length of your assessment and the time of the exam!), so gradually you can build up your memory, and you don't have to learn it all at once.
If I think of anything else I'll say... But these are the main things I do.
Hope this helps!