How can I become a specialist?

I have studied the following topics: - Basic Data Structures (STLs) - Number Theory (Sieve and Segmented Sieve, Factorization, Binary & Large Exponentiation, Sum of Divisors (SOD), Euclidean Algorithm, Modular Arithmetic, Big Integers, Big Mod) - Combinatorics - Algorithms (Prefix Sum, Two Pointers, Sliding Window, Binary Search)

Although I understand the concepts of these topics, I am facing difficulties in applying them to problem-solving. I am unsure how to leverage these topics effectively to approach and solve problems. Could you provide guidance on how to overcome these challenges and improve my problem-solving skills? Additionally, are there any other topics I should focus on to further develop my expertise and work towards becoming a specialist in this field?