Firstly, instead of saying you can't, you hat you can you just need to go about it a certain way. Are you taking your GCSEs this year or next year? I got a B in GCSE maths and I took it for AS (I'm still in AS). I dropped it a week before Christmas because I couldn't cope. Looking back now, I realised that I went about it the wrong way. If your school has a thing after school, I highly recommend you go to it. Do as many questions as you can and try learning general formulas that you can apply rather than learning how to individual questions. Also go on youtube and find a videos for each topic of GCSE maths. You could even go back to the very basics and re-learn the simple stuff. Don't get yourself in a panic because your a bit stuck, get your teachers help, if that doesn't work that you could try getting other students to help or get a tutor. The most important thing to remember is to remember you can do it, you just need more practise.