There's no point crying over spilt milk.
Similar phrase, similar meaning, perhaps easier to understand.
Spilling milk is a trivial problem. It can easily be cleaned up and replaced. There's no use in getting worked up over it, you should just carry on as normal, as it's not going to affect you any more.
The same thing can be said for "It's in the past".
It's in the past, so it's not going to affect you any more. Therefore, why bother getting worked up over it, since you won't be changing or preventing anything and you'll just be getting yourself angry.
Of course there are exceptions. There are things that are such an issue that they still carry on to be an issue in the present, or they caused you serious pain/trouble.
Or in regards to other people, their actions are likely to be repeated in the future.
That might be the distinction really, whether someone actively did something bad, or if they made a mistake.
Normally you can trust someone not to make the same mistake twice.
