From Longman Dictionary of Contemporary English

be (all) water under the bridge

informally used to say that what happened in the past should be forgotten

Take a look at this example:

from the movie Old School (2003)

Frank: So what’s up?

Marissa: Oh nothing much. I was kind of hoping we might get to do those thank you notes tomorrow night.

Frank: Oh honey, I’ve got Mitch’s thing tomorrow night.

Marissa: Oh that’s right. I totally forgot.

Frank: But I can skip it.

Marissa: Oh, no. No, don’t be silly. I mean, I wouldn’t want you changing your whole life just for me. But you know what, I’ll give Laura a call and we’ll plan a girl’s night. It’ll be fun.

Frank: Thanks. You’re the best.

Marissa: Just as long as you promise to take it easy, you know.

Frank: What do you mean?

Marissa: You know exactly what I mean. You’ve come a long way since Frank the Tank and we don’t want him coming back now, do we?

Frank: Honey, Frank the Tank is not coming back, okay? That part of me is over. It’s water under the bridge. I promise.