java - How to add external JPanel in JFrame?

I'm working on large scale program. As you can see I have one main JFrame and about 20 menu items on that. Each menu item must pop up a new window. At the beginning I have created a JLayeredPanel and then I assigned each menu item to one JPanel which is inside JFrame.Then I put 25 panel in JLayeredPanel... Default all the panels are set to invisible like:

panel1.setVisible(false);
panel2.setVisible(false);

so on

When user click on one menu item, its JPanel will be visible and rest are invisible. It looks messy and I have 5000 lines code. I used InternalFrame and TabbedPane but I'm not happy with them. I want to split my code in different JPanel classes and assign them to the main JFrame. I mean when user clicked on each menu item it will call the external JPanel and render it on the JPanel on the main JFrame. I am using design mode in netbeans and it does everything for me but the simpled structure is like this and it is not working:

 public class NewJPanel extends JPanel{
    //I have added buttons and etc on this panel
    ......

    }

public class  frame extends JFrame(){

        JPanel panel = new JPanel();
        .....

        Public frame(){
             frame.add(panel);
        }
        ......
        //When use click on the any button on the panel
        private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
            //this is not working

            NewJPanel fi = new NewJPanel ();
            panel1.add(fi);


            //or I tested this way separately but it did not work

           panel1.remove();
           panel1 = new NewJPanel();
           add(panel);
           invalidate();
        }       



}

please give me any suggestion how I can control this program in splited classes in professional way.