Have not tried this with bound form fields but here is the example I used to test the code.

1. Create a form called InstructionF
2. Create 4 unbound text boxes called NameTxt, DateOfBirthTxt, AgeTxt, GenderTxt. Delete the labels.
3. Create a label on the form called InstructionLbl
4. In the forms onOpen Event place the following code.
DoCmd.GoToControl "NameTxt"
Me.InstructionLbl.Caption = "Enter Full Name"
5 Create a private sub In the form called ChangeUserInstructions
6 In the GotFocus Event of the 4 unbound text boxes call ChangeUserInstructions
7. Place the following code in the ChangeUserInstructions private sub

On Error Resume Next
    Dim Ctrl As Control
    
   Set Ctrl = Screen.ActiveControl

   If Ctrl.Name = "NameTxt" Then
        Me.InstructionLbl.Caption =   "Enter Full Name"
    End If
              
If Ctrl.Name = "DateOfBirthTxt" Then
            Me.InstructionLbl.Caption = "Enter Date Of Birth"
End If
    
If Ctrl.Name = "AgeTxt" Then
            Me.InstructionLbl.Caption = "Enter Age"
End If
    
If Ctrl.Name"GenderTxt" Then
          Me.InstructionLbl.Caption = "Enter Gender"
End If