What concept marks your boundary of knowledge in your area of interest?

I posted this elsewhere today here on hacker news, but my Discrete math teacher once said to our class that to learn something you basically have to already know it so in that sense I see a lot of things as barely if not far out of my reach. I also understand he kind of meant by this that knowledge in theoretical fields like Mathematics comes in steps. So I never think oh I could never do that. Currently actually grasping linear algebra. I never took a course for linear algebra in my CS curriculum I chose the least math involved emphasis I could in CS and I turned out to regret it because once I got to the funner abstract math I actual found out I enjoyed it. I’ve felt Linear Algebra is just outside my reach so machine learning has been something hanging over my head for a while now, because I haven’t put my time into it yet because I haven’t meet the necessary prerequisites. But honestly I’m kind of just trying it anyways. I have a decent enough Math background at the moment that I can just go for learning machine learning but I haven’t yet. I keep circling around it but I think analysis paralysis is getting to me. Self learning is a bit hard for me when it comes to dedicating my time solely to one thing because there is so much I want to learn across the CS discipline.

The concept that I consider to be unachievable is being successful.