If something is already done and over with there is no point in still talking about it.
If something is already broken there is no point in trying to use it.
Relationship = dead horse
Bitching about it = beating it
Trying over and over to start your car after you've been in an accident where it is clear that the car is not going to work.
Broken Car = dead horse
Trying to start it = beating it
taken from the act of whipping a horse to get it to move, even though it is dead, and aint going anywhere.
"It's going nowhere, stop beating the dead horse"