What's done is done
Usage in the Play and Today
In the play, Lady Macbeth is consoling her husband, Macbeth, because he's experiencing guilty hallucinations after murdering King Duncan. She says to Macbeth that he should have stopped thinking about the king after he already killed him; he can't fix it now so he shouldn't give it a second thought. The phrase, "what's done is done" is used exactly in the same way as its used today—there's no changing the past, so forget about it.
There are many situations in which the advice "what's done is done" can be applied to, but break-ups are among the most common. After a break-up, friends would say something such as "I'm sorry things didn't work out between you and ______, but what's done is done; it's time to move on."