First of all you must really be working on some top notch "classified" project that you are feeling so vulnerable to divulge the details off. But just imagine how helpful we can be if we do not even know the problem domain for the project.
And also it isn't so bad to correct yourself if someone asks you to do the right thing the right way.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...