Yes, when it comes to calculus, I do require more than one textbook to understand the concept rather than just memorize it. I have a 4 year high school background in getting good grades for biology and mathematics/calculus, so it's really hard to figure out what skill I missed earlier in order to help me progress now. But I will take your advice for calculus, since I am not very happy with my textbook by Thomas. I am taking Calculus for the Life Sciences, where the material is much more condensed compared to other calculus courses offered.
As for biology, especially for genetics, it requires more than just reading and understanding, because there is a lot of information to process, and such details that dig deep into the core of DNA. Even the naming is based on codes that can overlap with other DNA processes, so if I'm not paying attention I could misread a gene transcription code, etc... It is a very competitive field, and I have to choose my words carefully for the short answer questions because they are compared with everyone else's answers to see whether we understand or if we're using the wrong word choice. So we can't always re-phrase or put the definitions in our own words. It almost feels like I'm studying how to write my answer, rather than actually writing down how I really understand something.
But anyway, I have been writing down questions formulated after reading a slide in the powerpoint lecture, or a page in the textbook just so I can tackle the answer in more than 1 way. I also stick with my T.As rather than the professors, since they are more experienced with common mistakes that students make. They are also more familiar with the kind of questions that will be asked, so I do depend on their advice. All this has only lead me to scoring between 65 and 70s, and I do see some slight improvements, but it's been two years, and no 80's yet.
I will keep trying different methods, so I'll see where it gets me, because I have no problem in understanding the material, it's just a matter of knowing how to present it. Maybe that's why I stress out a lot, because I feel like my effort and understanding is not being reflected in my marks. When I get a bad mark, I don't feel depressed or doubt myself for not knowing any better, I just get upset with how future employers, and other people will look at the marks and not feel the same. I have enough hands-on experience in the field, I just need to prove myself through my marks and that is what I am still trying to figure out.