im 17 re doing my GCSE maths AGAIN.

Hi im 17 and currently in college and i failed my GCSE maths and twice now.
I really struggle with school especially when it comes to exams especially maths!. When i was younger apparently the doctors wanted to diagnose me with ADD and maybe other things that would effect my capability of learning as i got older but my mum refused and never got me checked or diagnosed, and im just now realising if i went to the doctors about it maybe it would've helped me with my education as ive always struggled with learning throughout school even when i was really young. But its too late now. throughout school i wasn't really taught anything they just ignored me because they thought i was too dumb but in reality i just wasn't getting the support i needed. I remember in year 10 or 11 (one of the most important years) i got my mock grade back and it was the exact same as my best friends ( i think it was 3.8-4) but i got put down to the lowest bottom set.. when my friends were learning tin cos tan,alegebra etc i was given 'what's 5x2 and what's 2x5' and that really pushed me back because i knew i wasn't supposed to be in that class because i was always the first one to finish. And eventually just stopped going into school as it was such a waste of time.
i eventually got put back into my old class but it was too late cuz i was a already so far behind. then GCSE'S came and i got a 3.
Then it came to joining college. i had very limited options as i didnt find any interest doing anything practical. i prefer written work! i chose BTEC business.bad idea litteally got taught nothing they would just go wander off - i understand if theyre busy it just wasnt helpful lol.the teachers just had sm attitude.i failed one of my units that i really tried hard on and i wrote sm essays for it and it was my best work i went for the heighest grade too but i failed because my teachers pc wasnt working... and i even showed him that any other pc can open the document and he didnt care and he marked it as failed. my maths teacher was a really sweet old lady but she didnt teach me anything.so i got another 3.
This year im doing a different course and my maths teacher i really find her teaching helpful and ive learnt more stuff this year than i had in secondary school! but guess what i got a 3! im always like lessthatn 5 maarks off max 10 but i really find it hard to concentrate and revise does anyone have any tips for me? i really just want to pass but its so difficult because i cant remember anything - pls dont be rude just cuz ur blessed with being smart HAHA.sorry its a long msg!