Hey, I did my A Levels in 2019 and I can understand how you are feeling. Honestly, please do not try to stress yourself out too much because of course it is important to do well but your mental health also matters too and you just need to try as hard as possible.
One thing that might help with feeling less overwhelmed is to PROPERLY GET ORGANISED. Sort out the content e.g. two big ring binders for each subject (one for Year 12 and one for Year 13).
Make sure you have your specification printed off for each subject and USE THAT AS A CHECKLIST! It will help you visualise everything you need to get through in terms of content and it will put you more at ease. You can divide the specification into dividers in your folder.
Find a method that works for you to learn the content. Personally, I found that active recall was very useful for the science subjects - check the video out by Unjaded Jade. You basically blurt out everything on a paper and write down everything in a different colour that you forgot. For geography, I used flashcards with key points, definitions and case studies.
After you know the content, DO EXAM QUESTIONS!!! This is definitely what you should do the most - because you can take exam answers and insert them into your notes and memorise these.
Honestly, A Levels are definitely manageable if you are organised and you have PLENTY of time to do things right! Stay in a positive mindset and look after yourself, try not to burn out! You got this x