Oh dear, this thread is very topical for me.
I am a man whose partner of many years broke with me a month ago, but told me last Sunday that the real reason was she had taken up with an old ex.
He was a serious boyfriend when they were students -almost 40, yes 40, years ago- and he ended the relationship and quickly married another woman. My ex ended up marrying another, and was married to that man until divorce after 23 years when he was unfaithful.
My ex looked him her old boyfriend on Facebook last year, and started up with him. The man's wife recently came across the Facebook contact with my ex and threw him out of the house, so in a way it was forced on them but anyway I'm sent on my way.
I'm in a state of shock over this, it's non-negotiable as far as my ex is concerned because she says it's true love.
Her line is that her and my relationship was on the way out anyway, and her old boyfriend's marriage was hollow. It sounds like the usual line men take in similar circumstances, in fact a friend of mine (male) says she has acted like a man.