Managed C++ And C++/CLI :: Breaking Up Large Class Into Manageable Source Files?

I'm trying to make sure my code is written in smaller modules, so my first step is to create my initialization process in and external file to load the necessary data from external sources and set up things like the content of drop down list boxes.

My first attempt failed to give me access to the combobox items add function so I moved that code back into the form1.h file:

Code:
public:
Form1(void) {
InitializeComponent();
//
//TODO: Add the constructor code here
//
}
void AddDate(char *date, int ID)
{
this->comboBox1->Items->Add("line 1");
}

It compiles fine, but the call to it in my Initialize.cpp file

Code:
MarketView::Form1::AddDate("abs",1);
Gives error C2352: 'MarketView::Form1::AddDate' : illegal call of non-static member function

OK, so I change "void AddDate" to "static void AddDate" and now get the error that "static member functions do not have 'this' pointers" so I go back to the "MarketView::Form1::comboBox1" situation where there is no legal syntax after "Box1 to get me to Items->Add

I've been an old fashion programmer for over 47 years. It seems as is the concept of programming computers has changed from the concepts of logic to memorization of complex syntax.

There has to be a simple answer to do this other than to write thousands of lines of code in one Form1.h file. I refuse to believe that the new programming concepts will not allow you to write code in smaller more manageable modules.

What is the proper syntax for breaking up the larger file into more manageable chucks?