Quizá con esto te arregles:
Código vb:
Ver originalPrivate Sub RichTextBox1_GotFocus()
RichTextBox1.SelStart = Len(RichTextBox1.Text)
End Sub
Private Sub RichTextBox1_KeyUp(KeyCode As Integer, Shift As Integer)
RichTextBox1.SelStart = Len(RichTextBox1.Text)
End Sub
Private Sub RichTextBox1_MouseUp(Button As Integer, Shift As Integer, x As Single, y As Single)
RichTextBox1.SelStart = Len(RichTextBox1.Text)
End Sub
De ese modo el cursor siempre se coloca al final.
Saludos