In honesty, alot of the detail you find in textbooks you'll only need to remember for specific examinations.
It's alot easier if you break it into chunks and try to understand core concepts as oppose to memorise everything.
You'll notice that unlike at A-level, there are themes that occur throughout medicine, and as time progresses, you'll begin to integrate your knowledge of different systems and that makes it a hell of a lot easier.
For example- you could memorise that Right heart failure is most commonly as a consequence of Left heart failure and this leads to oedema in the peripheries.
Or you could work your way through it using what you know-
Left Heart Failure -> Pulmonary Congestion -> Increased Pulmonary Pressure -> Right Heart pumps harder -> Eventually fails -> Blood accumulates in veins -> Venous Congestion -> Starlings forces favour tissue fluid formation.
Medicine is hard, but once you realise that alot of the stuff is just the same core principles applied in different settings, it's not so bad. That being said, you'll learn alot of irrelevant crap depending on what med school you go to which will require some degree of memory, but it's manageable.