To be honest - get back to the GCSE books, and re-learn the topic more deeply. Anything related to question spotting will find you out - it's a good solid grounding you need. Maybe try some other sources: there are multiple ways to teach maths, and some may suit you better than others.
Very few people are genuinely bad at maths, but many have been poorly taught or have developed the belief that they can't do it because of external negativity.
G