Hey! First of all, this is super common, so don't stress too much about it!
My first question would be, how did you fare in those practice tests? If you are doing as many as you say and getting them all right under exam conditions, then you understand your stuff. There's no way you could answer all the questions under exam conditions if you didn't.
My next question would be, where are the questions coming from? You want to make sure they are of similar difficulty to what you'll actually be getting in exams - Trial/HSC papers are best for this! It might be that your practice just isn't quite testing you to the same level that the exam did.
If you were doing a range of tricky questions, and getting them all right, then your issue is with exam technique. This could be down to anxiety, or over-thinking, or any number of things - But if you get the questions right
out of the room then don't get them right
in the room, then logically, the issue is the room. This is a hard thing to deal with, but it comes with practice and learning to disassociate the exam with the result - No one would perform at their best if they spent the exam getting nervous about the significance of the result
I think it's just about being honest with yourself at every stage of the process - You are doing all of the right things! Perhaps reflect on where exactly you think the weak link is, and start there