It's water under the bridge.

Meaning

It's a past event that is no longer important.

Examples

• John: I'm still upset about what happened last year.
• Alice: I know, but <b>it's water under the bridge</b>. We should move on.
• John: I suppose you're right. It's difficult, though.
• Alice: Yes, but dwelling on it won't change anything. Let's focus on the present; <b>it's water under the bridge</b>.