How can I remember all of the content in A-Level biology?

A-Level Biology has probably the most content of any of the A-Level sciences! Its tempting to memorise all the content in the textbook but in reality some textbooks will have extra information that is not actually on the specification. What I would recommend is to print off the specification from your exam board website, and when you're planning revision, go through each specification point and make sure you've covered it.
Also try not to rely too heavily on memorising everything, if you find a particular topic difficult to remember, it could be better to try and incorporate more practice questions on that topic into your revision. The more questions you do, the more you'll remember the content, and the better you'll get at applying that knowledge in an exam.