Although you're only at GCSE, you probably do know more french than you realise and could improvise if you get stuck. I can't memorise anything so I learnt a few landmark phrases throughout my essay and just practiced improvising in between them. Landmark phrases are important so you don't get lost or waffle and you can make sure you're meeting all the criteria, I had the first word or two of each of these on my 40 word sheet. Splitting it into sections will definitely help too, things like writing it out in different colours to help you visualise it in the exam are helpful.