How to organise large code files?

Question

I am increasingly aware that my code in any single file can often span hundreds of lines quite easily and although I know the implementation might be sound, it still feels messy

Answer 1

If your files are big because they contain a single class/struct that is big, then this is usually (but not always) a hint that your class is dealing with multiple concerns and can be refactored into a number of smaller, more specialised classes.