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.