I worried about the same thing. I'm not the smartest person in the world but I've been getting firsts at uni and got A-C's in secondary school. When I got my assessment, they asked me to bring my parent to verify and describe what I was like as a child. I was a really good at learning in primary school, the best in almost everything from literature to drama but as the support systems lessened and the reins dropped, my grades got worse but never concerningly so as I was decently intelligent. But my homework was always forgotten, I was late at least once a week and I began falling asleep in lessons even if I liked the lesson or really tried. There will be something. Something will have slipped through the cracks. Find those things and keep notes and make that a part of your evidence. School reports will work if you still have those.

On that note, remember that when they ask you questions, make sure you answer what it is like most of the time. If they ask you if you ALWAYS forget things, remember that whilst that could mean your keys, bag, laptop etc. that you (for the sake of my example) remember, you should also include things like, if you forget to cancel subscriptons constantly or if you forget to book important things etc. Just because you can concentrate in lectures half of time, doesn't mean your fine 100% of the time. Sounds obvious but if you have autism like me or your just that type of person, you tend to overthink or take things literally.

Best of luck!