I worked through my prerequisites as well and it's just a matter of carving out study time wherever you can every day, even if it's just 15 minutes here and there. If your teacher provides lecture outlines use those as a jumping-off point for what to study in the book. Everyone studies differently but some of my go-to tactics are writing out definitions of key terms (instead of just highlighting), re-drawing and labeling important diagrams by hand (really helpful in understanding action potentials and cellular transport), making flow charts for things like hormone pathways and factors affecting cardiac output, and most importantly being able to construct and explain feedback loops from memory. My teacher was big on "telling the story" of each system and practicing feedback loops was probably the single most important step in understanding how all of the components of a system work together to produce a certain outcome.
Also, if you haven't already done so go talk to your teacher during their office hours and see if they have any suggestions for what you can do to raise your lecture grade. Good luck!