How to fill gaps in my math knowledge?

You want to look at one or both of the following, excellent, resources:

• Thinking Mathematically: By Mason, Burton, and Stacey. This will help tremendously with being able to approach problem statements "mathematically", for example, how to deal with, think about, or read, mathematically, "word problems", and problem statements, break them down, approach them, and solve them.
• How to Prove it: A Structured Approach: by Daniel Velleman. Excellent survey of logic, proof strategies, and proof-writing, with many hands-on examples and practice problems to develop these skills.

Both will be excellent preparation for and throughout college.

ADDED:

• Another "classic" is Polya's book How to Solve it. It's a gem for better understanding and developing the skills and strategies needed when in problem-solving.

Also, for practical purposes: you might want to take a look at Paul's Online Notes, for some pre-college and "undergraduate level" lecture notes and coursework, compiled and maintained by Paul Dawkins, from years of college teaching at Lamar University.