I'm in the same boat as you so I know how you feel, what's your exam board? I study AQA. Given the amount of content & memorisation of the information needed for your exams I would really advice you to put some kind of action-plan together? For example, revise one biology topic such as transport across cell membranes on one day, and on the next day revise biodiversity etc. If it's a big topic, like cells, maybe spread that over 2 days and return to it after some time to ensure it stays fresh in your memory?
If you can split all of the content into smaller sized chunks and focus on each chunk individually, you won't feel as overwhelmed I think. It's worked for me, I went from an E to an A/B.
Use flash cards for definitions too! They are so useful.