Once by my ex-fiance because I found out the charming person I fell for was a lie. I still stayed with him for almost two years before the abuse was too much. I'm quite happy to have no contact with him at all (and haven't since we parted ways except for a period of time where he stalked me and one other time when he contacted me).
Twice with people I met online whose words were nothing but lies. People too cowardly to ever meet and give things a chance. I still hurt over both of them and mourn what might have been.
Once when I was still in the hospital after delivering my son and seeing his adoptive parents walking away with him. I cried myself to sleep nearly every night for a year following that.
Throughout all the cruddy things I've endured/lived through, I've learned a lot. I don't want to repeat any of it, I have things I regret in my life, but I value the things I've learned.