Since Trim only removes from the beginning and end, the whitespace character at the end doesn't match anything in your char array so it's done trimming. If you are trying to remove characters from the string, then you need to use the Remove method.
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 ...