Water under the bridge.

Meaning

Something that happened in the past that is no longer important or relevant.

Examples

• John: 'I still feel bad about that argument we had last week.'
• Alice: '<b>Water under the bridge</b>, John. Let's focus on the future.'
• John: 'It's <b>water under the bridge</b> now, we've learned from it.'
• Alice: 'I'm still a bit apprehensive about that past project failure.'