It does seem common to forget most of each course. At least that was the case for most of my students for 30+ years in university classes. I eventually learned to teach essentially every class from scratch, as if the students remembered virtually nothing from previous courses. Even when teaching graduate PhD candidates, I always had to review the two most basic theorems of advanced calculus, the implicit function theorem and Green's theorem.
This always puzzled me because I had not forgotten myself essentially anything I had learned in high school math classes for my whole life. I had participated in math contests regularly, stayed after school to practice for them, and probably benefited from that extra practice at learning reinforcing and using the material.
But I am convinced my students' forgetting showed that many of them never really learned the material well at all. Basic study habits such as reviewing the material every day after class, would have made impossible the sort of situations I faced every day. Other students made it clear to me that they never studied at all on weekends, regarding them as purely for social activities.
Office hours were routinely ignored, no questions asked in class, and questions were never raised until the day before, sometimes hours or literally minutes before a test.
Some students seemed to think that the prerequisite for a class was simply to have taken the previous class even getting a D in it, rather than to have learned and retained the prerequisite material. You would think it obvious that if algebra is a prerequisite for calculus, and a student has forgotten all his algebra, that he would realize he does not have the prerequisite, and that he would review algebra in the summer perhaps before taking calculus. However this seemed uncommon behavior.
Perhaps some of my students, certainly not all, thought that the only purpose in taking courses was to get a degree, rather than to learn what was taught in the courses. Perhaps some thought that a degree, rather than knowledge, was the key to a good job. Some students focus entirely on passing the class rather than learning the subject. For this reason some seek out the easiest teacher,f rom whom they will learn least, rather than the most demanding teacher, who will push them the furthest.
This may be partly the fault of those of us who teach without giving any indication of how the material we teach will be used. If we tell students why they need to know something, maybe it will help. [this has been edited.]
By the way, the fact that you are asking this question shows that you are motivated to improve your study skills. More power to you.