It is perfectly normal for humans to forget things after a period of time. However, here are some tips to help you "relearn" the material you have forgotten.
From your question, I understand that most of your learning came from KhanAcademy. While KhanAcademy is a great learning tool, it was never intended to replace a full year-long (or semester-long) course. Instead, it is mostly used as a supplement to reinforce concepts learned in the classroom.
Khan Academy is a great tool; however, it should not comprise the majority of your work on a subject. If you still have your high school textbooks, dust off the cover and reread them. Do the exercises at the end of each chapter - I know most textbooks have answers to odd-numbered questions. If you can't do a question, ask for help (like here!)
If you don't have them anymore and you don't want to buy a $200 textbook, you can get free exercises online. Just search up, say, "trigonometric identities practice" or "polar equations review problems". The more math you do, the better you will become.
In the beginning, it will be tough. You might be frustrated, and it might seem like you aren't making any progress. However, it will get much easier as you progress, and you might even find yourself enjoying the problems that you are doing.
But no matter what, don't procrastinate! The sooner you start, the more you will learn. And the more you do, the better you will get. You shouldn't have to memorize formulas by heart, strive for understanding, not for memory. For example, consider all those complicated trigonometric identities like $\sin(x+y)=\sin x\cdot\cos y+\sin y\cdot\cos x$ Do not memorize these formulas; instead, understand why they are true. Only this way will you be able to remember them.
Good luck, and have fun with the problems! :)
Best Answer
One place in statistics where abstract algebra (specically finite fields) is used is in the construction of mutually orthogonal latin squares which are used in Design of (statistical) Experiments. Knowing linear algebra, matrix, eigenvalues, positive definiteness, generalized inverses is the most important thing. You can safely skip abstract algebra and focus on applications of matrices. One small book that does this well is by Bapat (Linear Algebra and Linear Models?) published by Hindustan Book Agency, New Delhi.