С++ GUI wxWidgets: Code Architecture, Design Principles and Patterns

I recently started learning C++ and wxWidgets. Now I'm doing a calculator. When I start writing code, everything is ok, but when code gets bigger and bigger, it becomes a mess.

One thing that I realized, and that turns my code into a mess, is that I'm putting all my code into one file instead of splitting it into several files.

I want to know any C++ GUI code architectures, design principles and patterns that will help to write clean, flexible and scalable GUI applications.