Five months and three weeks can mould a positive outcome with your GCSEs if you spend the remaining time wisely and effectively. There are many personal stories online, of how individuals with similar experiences were able to flip their grades achieving high marks with their limited time, I'm sure it's a task you're capable of too.
Sciences and maths are typically the subjects most students have trouble revising, you're not alone! I think it's important to figure out which type of learner you are, and what methods of revision suit you the best. What revision techniques have you tried so far? If you have tried a few of them, try to identify which one resonated with you the most. Mind maps? Flashcards? Pomodoro technique?
From personal experience, I've always found practicing past papers to be the most efficient for me. Past papers allowed me to familiarise myself with the types of questions I can expect, which topic areas I need to improve, and helped me to apply the pre-existing knowledge I gained from revision into context. Another tip I adopted while doing past papers (typically for the sciences) was to sought out for those large 5-9 mark questions and write it on a flashcard. On one side I would have the question, and on the other side I would have the mark scheme. Often times I would try to paraphrase it without straying too far from the answer to help me remember it in my own way and strengthen my understanding. This technique was extremely useful for my biology papers.
I don't believe your goals are too ambitious, they are definitely achievable with the right grit and diligence. If you want to read more about tips and additional revision techniques, we have a more coherent and in-depth guide here
Best of luck!
BCU Student Rep