Can you ask to do a trial period which is basically when you study a subject and if you get over a percentage (e.g. 60%) in the first exam, the teachers let you stay on that subject? Can you ask to do that with bio and chem? Also, don’t study those subjects to prove something to your parents or you will spend 2 years studying subjects that you hate. Instead, do it for you and they will be happy. Plus you could always do a master's degree in the uni you aspire to go to. Ask your school if you can change your AS grades after November mocks. Or you can start year 12 again and you won’t have to worry about being behind as a lot of people take gap years etc. Try Maths GCSE tutor on YouTube, it’s like a walkthrough and shows how to do the questions. Also, comparison is the thief of joy, if you focus on doing well and getting to where you want to be, you’ll feel happier. At this age, a lot of people are in different stages of their life, you’re not behind and at least you’re making an effort and preserving. It shows dedication. Good luck ♥️