I graduated first in my class at LSU in 1989, summa cum laude with a Bachelor of Science degree in Physics.
It is normal to forget without constant use or some kind of reminder. Our minds are fallen, and without constant use and reminding, skills and information quickly become rusty and decay.
In my college days, I used layered techniques to help put things more reliably into long term memory. The curriculum and assessment methods of my professors were also structured to help that.
I took careful notes in every class. Since these notes tended to be messy and disorganized, I recopied a more organized and neat version of the notes shortly after every lecture while the lecture was fresh in my mind. Every day, every class, every year. Writing things down and then recopying them is a great memory tool.
I worked every assigned homework problem. Since my original written solution was often messy and disorganized with erasures and less than a linear progression (blind alleys and so on), I recopied my solution very neatly before turning in the problem set. Preparing a neat copy of every problem before turning them in had the effect of better cementing the solutions in my memory.
As each test neared, I re-read the material in the book, reviewed my notes, and selected a subset of the assigned homework to re-work in preparation for the test. About half my time in test preparation was dedicated to reworking homework problems (without looking at original solutions) as practice to recall how to do them.
As the final exam neared, I began my preparation about a week or two beforehand. I would re-read the most challenging chapters, re-work all the test problems I had missed, pick new problems from the test to work, and prepare for myself a “practice final exam” from which I drew about twice as many book problems as problems on most tests, put myself under time pressure and the authorized resources of the real exam, and did the best I could. After the allotted time was over, I spent as much time as needed to work each problem correctly (visiting professors during office hours as needed), and assessed how prepared I was by how I did under pressure of time and authorized resources.
On the whole, my preparation required 2-3 hours of real hard work for each hour I spent in class. Most of that time was spent with my pencil moving and my mind fully engaged. No mind wandering. No distractions. No Facebook, phone, or TV. In total, college was a 60 hour per week full time job for a 14-16 credit hour course load. The reward: I graduated first in my class, summa cum laude, with a 3.95 GPA and admission and fellowship offers to MIT, Standford, Princeton, and Stonybrook for graduate school. I was done washing dishes in seedy New Orleans restaurants and flipping burgers in fast food joints.
Subsequent courses tended to reinforce pre-requisite material from earlier courses through practice and repetition in the assigned homework. It was my Calc 1 course where I practiced enough algebra to finally have those techniques cemented into long term memory along with really understanding what a function is. Calc 3 finally cemented many ideas and techniques of Calc 1 and Calc 2. The upper level E&M sequence cemented many ideas from 2nd semester freshman Physics, and so on.
Still, when I got to MIT (graduate school) I spent most of my first year re-taking key undergraduate courses, including Mechanics, E&M, Statistical Mechanics, and Quantum Mechanics. Forgetting is normal. Mastery and longer term learning requires repetition.