I am in Year 10, not doing Spanish but I am doing French. I find that for speaking assessments particularly the best thing to do is to make sure you know exactly what you are supposed to be talking about - that way even if you can't remember the exact wording you can attempt to come up with something along the same lines, instead of panicking.
I'm assuming it's the same for Spanish, but in french we're allowed a certain amount of words to use on a cue card, so for this I'd recommend writing down the words you struggle to remember. For the cue card I often use different colours for different topics etc, as this helps you to remember what you're meant to be talking about. I'd recommend carrying around the cue card for a few days prior to the test, just so you can go through it every now and again, as this helps it stick in your mind.
Overall, if you have a specific topic to talk about in the assessment, find a way to link it to yourself, as this will help you remember it. For example, if the topic is what you did last weekend, don't just make something up, try to include what you really did do, because even if you can't remember exactly what you wrote you'll remember what you did, so you can attempt to wing it!
I hope it goes well for you