I think what happens is that in time the person left realises that life is "bigger" than them and there is so much more to learn, love and enjoy than that one person.
My world is so much larger now that I have divorced my cheating ex husband. I thought I couldn''t cope without him, the children would be devastated, finances would be tough, etc etc.
But of course I can and do cope, very well, the children have been devastated at his attitude towards them, but have now just accepted that he doesn''t want to have a relationship with them and basically have got on with their lives without him in it, finances are hard, but not as hard as they could have been, i.e. he stopped paying maintenance, but I took him to court and now money is sorted.
Sometimes we revolve our lives around our partners and when they leave we are lost and do not believe their can possibly be a life without them.
But of course, as so many wikki members are testament to, lives can be so much better.