Hi! First of all those marks aren’t bad at all, it shows you have got a good solid knowledge of basic maths given that you scored well in the first half. The second half is where all the more difficult questions arise, ones that require more thinking definitely.
My advice for you would be to do a couple past papers and note down any topics you performed badly on. Then revise these particular topics using YouTube videos online and doing lots of past practice questions ( there’s many available online but you can especially use physics maths tutor!)
The thing with maths is just practice practice practice… and practice again.
And do not worry that you don’t have enough time! I sat my gcses a few years ago now and I was stuck on a 5 even up until the beginning of April. I ended up getting a 7 🙂 which I was very proud of. So you do have plenty of time, just use it wisely!
Good luck!