okay, first off, don't accept that you can't get a 7, just accept that it's going to be a lot of work
secondly, find a website that has your gcse spec, and cross off the topics that you know. then you know the ones that you need to learn. i'm really sorry that your teachers aren't that good, but feel free to reply to this at any point and i can explain things!
past papers only work when you know the content, but need to apply it to exam situations. by the sounds of it, you just need to learn the content. also, a five isn't rubbish! a 4/low 5 is average for the whole country!
we can work together to get you closer to that 7. as a grade 8 maths student, i'm happy to help whenever, and i'm fairly confident that i can explain things fairly well
i hope the rest of your results go well!