Don't worry about developing. Maths is the easiest subject to get better at, especially with time and practice. Trust me - I'd know. I went from not knowing the area of a circle in year 7 and getting poor grades (5th lowest my in year 7/8 class) to now in year 11 and taking STEP (and AEA/M/FM).
Practice is key but also tutoring others. Tutoring others helps you to understand a concept to its simplest level and if you can teach something, it is then that you truly understand it. Teach others or pretend to teach others and talk to yourself (weird, but it helps). Here's a little routine I give to follow year 11 for Maths GCSE (this can be extended to Maths and Further Maths A level but not STEP/AEA as they're vastly different in question style).
For every topic:
- watch Mathswatch video on it
- watch examsolutions video on it
- watch random videos on YouTube to learn advanced examples of it
- go to CGP revision guide page for that topic and do the questions on there (the example and the ones on the bottom)
- go to the CGP workbook page for that topic and complete all questions for that topic (check your answers)
- go to the CGP Grade 9 workbook pages for that topic and complete all questions for that topic (some questions are quite fun)
- go to mathsgenie and do their worksheets for that topic
- go to justmaths and do their worksheets for that topic
- repeat for EVERY topic in Maths GCSE
- sit every since GCSE past paper (~past 7 papers) for (AQA, Edexcel, OCR; all 3)
- sit the CGP 9-1 practice papers (buy them)
- sit the Churchill 9-1 papers
- sit the CrashMaths papers
- sit the specimen papers
- sit all the practice set papers (and previous mocks)
I assure you, if you do this all properly - you will have NO trouble achieving 100%. This may actually be overdoing this but in doing this, you will master every topic and understand it to the fullest. Also, doing all the past 7 papers for all 3 exam boards (AQA, Edexcel, OCR) is very useful (even though the board may differ for you and it's the old spec; you should still do it). Completing the CGP papers and marking it will give you the grade you're working at (they have their own boundaries). CrashMath and Churchill papers are harder than normal papers therefore if you practice them, it's quite useful.