I agree with RunningScared that it is the memory of the 'Good times" t hat m akes grieving sooo difficult.
I am thiinking that the good times is what had us in the relationship, in the first place.
Not that there is abuse in every failed relationship, of course----but, I can see the parallel to this in the classic "Cycle of Abuse". The thing that keeps a person so stuck in the Cycle is the good parts, between the times of abusing. If the perpetrator was abusive all of the time---more people would run straight away!