I think MIT Open Courseware would be worth exploring.

Are you worried about getting in or are you worried about what you'll do when you get there? What I did was find a syllabus for one of my first year courses and then obtained the text book by searching online (this site is pretty good http://libgen.rs/) or checking the library. Don't worry about getting the exact version the point is to get a leg up on the concepts.

Khan Academy

If you're looking for books, then it depends on what field you want to study. Generally just I'd search for recommended textbooks for that field and then I'd definitely buy it and wouldn't just download it from libgen.is

I studied engineering after failing high-school algebra, so I was also pretty behind.

Khan Academy is a great resource for everything from elementary algebra to calculus and linear algebra, all of which you'll use.

Symbolab is great for figuring out tough homework problems.

YouTube is good for certain topics, and some may be able to explain how something applies to comp Sci.

Once you're in, use office hours. Some places have free peer tutoring. Everywhere has tutors you can pay for.

3Blue1Brown on youtube has amazingly good visual explanations for various math concepts. Helped me out a lot when I was having trouble with calculus. It doesn't help specifically with memorizing theorems or anything, but provides a good conceptual framework to start with. https://www.youtube.com/c/3blue1brown

~~Khan Academy~~

Edit: nm, didn't see you already use it

Paul's online math notes are by far the best resources out there for the first few university level math courses.

https://tutorial.math.lamar.edu

It is a great shame that his linear algebra notes were removed years ago.