How do I make my ex regret leaving me for an older coworker?

I've had been with a guy for over 4 years and a live in for the last 6-8 months leading upto breakup and i found out he had a relationship with a woman 6 years elder to him ( his Assistant manager) and he left me for her. I told the woman the same, she was shocked but took the boy back anyhow. " How do I make him regret this real bad for leaving and what exactly would you as boys think of such a girl had you been in his position?"
How do I make my ex regret leaving me for an older coworker?
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