You cannot convince that person that would be like forcing them to realize that you two were meant to be. If the other believes it then it was meant to be, if not then it wasn't.
You can't really they are going to believe what they want to believe no matter what you say. But I can tell you it doesn't help to be defensive, that will make it worse. You could just seem to have...
Well if you cheated then you need to tell them the real reason why you did what you did. If you dumped your bf/gf and now you want them back tell them how special they are to you and that they...