For me OO didn't 'click' until I read a book about design patterns. If you're already comfortable with concepts like abstract classes, interfaces, etc that's only half the battle.
The next step is figuring out why you should prefer composition over inheritance, how to code to an interface and how to write your classes so that they are decoupled and well-encapsulated. Design patterns show you solutions to common OO problems and help you structure your code adhering to the above guidelines.
I can't recommend any particular books about C++ but the GOF book is the standard on Design Patterns (Java). I prefer books that talk about design patterns in a particular language so that you can get concrete code examples. Design Patterns In Ruby is pretty good, as is PHP: Objects, Patterns and Practice.
I get the feeling that your instructor doesn't particularly know what he's talking about. 'More classes' is pretty useless advice by itself.
