How to clear textboxes nested inside a tabControl, tabPage and 2 panels?

The picture below represents a plugIn I am building.

Yellow = tabControl1; Orange = tabPage1; Blue = Main Panel (mainPanel); Green = 3 different panels inside the main panel (panel1, panel2 and panel3); White = TextBoxes (that I want to clear); DarkBlue = CheckBoxes corresponsing to the green panels

I am trying to clear any text from the text boxes if they have text in them and clear the checkboxes if they are checked with a button click.

I researched online and tried to accomplish this by the following way but it doesn't seem to be the correct way to handle this problem.

foreach (Control a in tabControl1.Controls)
            {
                foreach(Control b in tabPage1.Controls)
                {
                    foreach (Control c in mainPanel.Controls)
                    {
                        foreach (Control d in panel1.Controls)
                        {
                            if (d is TextBox)
                            {
                                d.Text = " ";
                            }
                            if (d is CheckBox)
                            {
                                ((CheckBox)d).Checked = false; 
                            }
                        }
                    }
                }
            }

I have only shown panel1 here but tried doing the same thing for panel 2 and 3 as well.

What exactly am I doing wrong here? Any help would be greatly appreciated!