you need to start with a change of mindset. Just because you will never be the smartest doesn't mean you shouldnt work. Also, trust me, not doing further maths GCSE will not hinder you future at all, provided you do A level further maths. Obviously its important to keep working, and you know that, but we are all finding it difficult and everyone has lost motivation at some point. I myself am just coming out of a motivation slump - I narrowly missed an offer from cambridge and I preceded to waste 3 weeks doing f all. When I decided to get back to working, it was hard, and I found that I just needed to tough it out before I could get into my normal flow of work. I have to keep building day by day - currently I am doing about 3 hours of independent work on top of lessons but in my peak its about 6.
There is a saying that motivation is a myth - what you need is discipline. I generally agree with this. If you can force yourself to sit down and start work, you will build up a mindset over time in which you are naturally motivated. Basically what I mean is just force your self to work at the start and you will get into a natural rhythm soon.
There are obviously little tricks which can help you, such as planning
exactly what you are going to the next day beforehand, moving your phone about 5m away while you work, making the working environment nice - Iv started turning lights off and using candles, and putting a video of a fire on my TV - weird I know but its nice.
But the more important thing is knowing how to study. I think you can definitely do well in every subject with less effort - even chemistry - if you know how to study. Im not sure if you have come across efficient study techniques in YouTube videos before but if not, here -
https://www.youtube.com/watch?v=ukLnPbIffxE&t=7s Basically the main point is that you should use revision methods which involve your brain actively recalling information to answer a question - this increases memory retention a lot. This means that making notes, reading notes and highlighting notes are ****. The best thing for GCSE as well as A level is doing practice questions. I got all 9s and 8s at GCSE and am on for 3 A*s at A level and by far my most important advice is just do every practice question you can find. Do it for all subjects, especially chemistry. There are plenty of question banks and past papers on the internet. Do the questions, mark them and write out the corrections on the question, make a note of your weak areas (I keep a note of this in my exam plan) and learn from it. The more questions you do, the better it will get.
Not only is making notes really boring and probably causing your lack of motivation, it is also wasting a lot of time and energy. Just focus on active recall stuff e.g. questions and flash cards.
If you already know about this stuff, then half of this isn't too useful for you but I hope it helps some how