Academic comeback

Hi , i have been revising for months for my maths exam and i still came out with a 4 or 5 , i done allot of past papers and i achieved 7s and 8s in them, i am really confused as of why i got so low on the mock exam. Ideally I want to get a grade 7 but i dont think it isn't possible. I can do the higher topics or question its only on exam days or weeks i forget how todo things I make ALLOT of silly mistakes that cost me marks aswell.
If you have any advice reagrding this comeback please let me know!

From what I can tell, it's likely nerves.
Did you do your past papers under timed conditions? If you haven't, then it's worth practicing under timed conditions. When you're under pressure, you tend to work and think differently. Getting used to the time restrictions and practicing effective exam techniques and using effective exam strategies tend to help a lot.
The other thing I would recommend is meditation. The idea behind it is to develop your sense of focus. Typically 5-10 minutes a day can do a lot.
One other thing that I could only recommend prior to sitting the actual thing is to do a past exam question (a 10-25 mark question; something that needs to get you thinking and getting into rhythm) about 15 minutes to 1 hour prior to the exam. The idea is to get your brain warmed up (if you're like most of us, you get brain fog in the morning and need some time before your brain gets into gear) and get into flow (google "flow state" for more information) before you head in for real. Once you open the papers, you want to be going in full throttle as opposed to getting a slow start,
For the time being though, it's really worth the time and effort to get a copy of your mock answer paper and see where you went wrong. Learn from these mistakes, formulate strategies to avoid repeating the same mistakes, and redo the paper (especially if you got specific answers wrong).
If they don't give you a copy of your mock paper for whatever reason, try to find a copy of the paper that you did from the exam board's website (it's usually a past paper). Try to remember what you did and what answer you gave for specific questions. Look at the answer paper, and try to learn where you went wrong.