Just because you wouldn't remember how to tackle an advanced math problem doesn't mean it has been wasted.

You've gained experience and you can identify what you don't remember. Even if you can't remember it it is no longer magic, which is invaluable by itself.

Also the process of learning these abstract concepts is arguably the most important skill you learn in higher education. You might recognize what is required and you do know how to pick it up when you are faced with it - or know when you need external help.