I study a lot but fail anyway

So i donno why that happens but i learn so much for my math exams, and while learning i do understand the topics enough to even pass. I also go to a private teacher who explains me all the stuff. But even then i cannot pass math. I do stupid mistakes. Sometimes i think i really have adhd. I think of the right number but write down a whole another thing. Or like i forget the things i learned w my teacher some couple of times. Can you guys maybe give me advice on how to nit make this happen again? And why dies this happen?