Is it perfectly normal to forget the maths/formula learned in High School? I am an adult learner, revising my high school mathematics from pre-calculus to calculus. I never took calculus in high school - trying to self-learn it. I was never good at mathematics.
I revised  Algebra 2 and Pre-Calculus few months back, mostly via KhanAcademy, watching some videos and completing some exercises. Now while learning Calculus, I've been unable to recall/grasp some of the concepts in Algebra 2/Pre-Calculus. I had to revisit those lectures and exercises multiple times.
Is this perfectly normal to forget High School mathematics after sometime? How can I really master the concepts and remember "formulae" by heart?
How do I improve on my understanding on maths and speed up the progress?
 A: 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! :)
