As I re-read the OP’s original post, I wondered if the Quarter System Schedule is part of the problem for the OP. For some students it is too much of a rush; too brief of a time to digest the data and concepts. Not true of everyone of course, but for anyone whom is not diligent in his/her STEM studies the condensed workload/schedule can be difficult.
Oregon State University and Louisiana Tech University are on the Quarter System.