I am using card layout and trying to swap panels but they are not swapping

See more:

Java

Swing

Later I want to pass NextPage information from Main Panel such as a string entered in a JTextField, but for now I am trying to swap between the two panels, but they are not responding. I want to keep my Controller panel as type JPanel as I will be incorporating into a tab later, I want to swap between Main and NextPage using buttons on those specific screens, I don't want to have consistent buttons on the bottom for both screens that switch between cards(i.e I don't want to have add & back to be appearing on both screens), I am trying to get the add button in Main to go to NextPage and back button in NextPage to go to Main. This is what I have so far:

Controller:

import java.awt.CardLayout;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Controller extends JPanel {

    private static Controller instance = new Controller();
    Main mainPanel;
    NextPage nextPage;
    private CardLayout cardLayout;
    private JPanel cards;

    public Controller() {
        cardLayout = new CardLayout();
        setLayout(new BorderLayout());
        setSize(810, 510);
        cards = new JPanel(cardLayout);

        mainPanel = new Main();
        nextPage = new NextPage();
        cards.add(mainPanel, "MAIN");
        cards.add(nextPage, "NEXT");
        add(cards);
        setVisible(true);
    }

    private static void createAndShowGUI() {
        JFrame frame = new JFrame("MainPanel");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Controller con = new Controller();
        frame.getContentPane().add(con);
        frame.setSize(800, 600);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
   

    public void changeCard(String card) {
        CardLayout cl = (CardLayout) (cards.getLayout());
        cl.show(cards, card);
    }

    public static Controller getInstance() {
        return instance;
    }
}

Main:

public class Main extends JPanel implements ActionListener {

    private JButton search, add, delete;
    private JTextField textField;

    public Main() {

        search = new JButton("Search");
        add = new JButton("Add");
        delete = new JButton("Delete");
        textField = new JTextField(20);
        add.addActionListener(this);
        delete.addActionListener(this);
        setLayout(new BorderLayout());
        JPanel top = new JPanel();
        top.add(search);
        add(top, BorderLayout.NORTH);
        JPanel bottom = new JPanel();
        bottom.add(add);
        bottom.add(delete);
        add(bottom, BorderLayout.SOUTH);
       // setVisible(true);
        setSize(400, 500);
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == add) {
            Controller.getInstance().changeCard("NEXT");
        } else if (e.getSource() == delete) {
           System.out.println("do something");
        }
    }

}

NextPage:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import java.util.*;

public class NextPage extends JPanel implements ActionListener {

    private JButton back;
    private JTextField textField;

    public NextPage() {
        back = new JButton("Back");
        textField = new JTextField(20);
        back.addActionListener(this);
        setLayout(new BorderLayout());
        add(back);
        setVisible(true);
        setSize(400, 500);
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == back) {
            Controller.getInstance().changeCard("MAIN");
        }

    }
}