Which character of Shakespeare said: "Things without all remedy should be without regard. What's done, is done"?

The phrase, "Things without all remedy should be without regard. What's done, is done" was used by Lady Macbeth in Act 3, Scene 2 of the tragedy play "Macbeth". In a key scene, Lady Macbeth directly encourages her husband not to continue thinking about his past but to only think about his future. She has asked Macbeth and his friend (Macduff) to apply her philosophy ("what is done, is done") as the preferred way to look at certain very distinct and important aspects of life. Her philosophy will make their lives successful.

Macbeth has to learn to be like Macduff. Macduff's focused attitude and determination in the wake of receiving very bad news demonstrated how he clearly applied the advice he obtained from Lady Macbeth.