I tested this and it worked for me. You could probably do a better job of scrolling by refactoring your Instr function. Maybe getting an absolute position vs. a carriage return.
Code:
If Count > 0 Then
'~~> Replace Textxx with the name of your textbox
' If Screen.ActiveControl.Name = "txtSelText" Then
If InStr(Me.txtSelText, vbCrLf) + 2 < Len(Me.txtSelText) Then
Me.txtSelText.SelStart = Me.txtSelText.SelStart + InStr(Me.txtSelText, vbCrLf) + 2
End If
Else
' If Screen.ActiveControl.Name = "txtSelText" Then
If InStr(Me.txtSelText, vbCrLf) + 1 < Len(Me.txtSelText) Then
Me.txtSelText.SelStart = Me.txtSelText.SelStart - InStr(Me.txtSelText, vbCrLf) - 1
End If
' End If
End If