Sometimes the book your prof is using just plain sucks. This may be the case for you...it may not come as easy to you as others but that's ok, you just might have to work harder for it. I still have my Bio 101 book and used it for more advanced classes to brush up sometimes all throughout my undergrad. My advice would be to try and study in groups b/c sometimes when a classmate explains things it is easier to understand and it benefits the person teaching you as well. I knew that if I could explain concepts to someone else that I had actually learned them and it was a form of review for me.
If that doesn't work...use the internet, don't waste money on other books. If you don't get, say for example, Meiosis, then google it and I'm sure you can find free tutorials. Hang in there, learning how to study and manage time is half the battle to succeeding in any professional program. As a freshman and sophomore that was my biggest challenge...all I cared about was where we were drinking and what girls were coming over...eventually you grow up, even if you don't really want to.