This is a reminder to not focus on the negative in your past and instead focus on the positive you want to head towards in your future.

Definition from this website quoted below: https://dictionary.cambridge.org/us/dictionary/english/cry-over-spilled-milk

to feel sorry or sad about something that has already happened; used to emphasize that this is not helpful:

It’s no use crying over spilled milk – he’s spent all the money, and there’s nothing you can do about it.

It’s a waste of time crying over spilt milk, he says; you just have to move on with your life.

Here we are crying over spilled milk when we should be thinking about the future.

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