Ok fair enough if you've been assessed - just a couple of other thoughts, at the risk of sounding like I'm trying to convince you - I'm not:
There's a reason women are mostly late diagnosed - the diagnostic criteria mainly focuses on how ADHD presents in males, who are more likely to be outwardly hyperactive - honestly it's kind of measured in how much they disrupt the people around them. Most women are more likely to present as inattentive ADHD, where the hyperactive part can take the form of constantly racing thoughts with an inability to switch them off, short attention span, difficulty in focusing and settling down to a task, particularly if it's one that is boring or difficult and doesn't trigger a hyperfocus - such as you and your year 9 curriculum.
Additionally, most women are late diagnosed because yes, symptoms do need to be present in childhood but when you're a child and most decisions are made for you, it's kind of easy to just mask. The signs might be more subtle if you're not bouncing off the walls and your hyperactivity is staying inside you as anxiety.
When you grow up and have children and a job, the mask gets harder and harder to maintain until suddenly, you're not coping. And then you blame yourself. That's how it was for me anyway. When i went back through my old school reports, the signs were there even though i was the quiet girl in the corner who never caused anyone any problems - i was scatter brained, couldn't focus, would dash off my homework at the last second after spending hours avoiding it or staring at it. I could revise for an exam for hours but nothing would go in. i internalised everything and that's what led to me being where i was mentally at age 35.
There are other neurodivergent conditions too. But if none of that resonates, please do ignore me. It's just that i read your posts and there's so much in there i can relate to - especially sitting down and forcing yourself to do a task that HAS to be done, but then just sitting there for hours not doing it - which makes you feel even worse about yourself.
Even if you're not ND, some of the tactics ND people use to trick themselves into getting stuff done might be helpful and alleviate some of your stress and anguish over it all. For eg with your year 9 curriculum, ive accepted now that stuff like that will be done last minute. I need a sense of urgency to get it done. I'll work out when it absolutely HAS to be done by, and try and break the task down into much smaller chunks, and I'll plan breaks in between - even if i only got 5 minutes work done then have a break to get a cup of tea. I'll have a familiar tv show or music in the background to help me focus. Sometimes i need silence. Sometimes i need to sit quietly and work out what exactly is it about the task that's got me stuck in task paralysis so i can address that first.
Basically - it's easy to blame yourself when you're not coping, but what if you're trying to cope with more than it's reasonable to expect any human to handle based on their capacity?