water under the bridge | water over the dam
Meaning: You can say a problem or an experience is water under the bridge, or water over the dam, if it happened in the past and it no longer affects the present to a degree that is worth worrying about.
- I wish people would stop talking about my problems with the law. It all happened a long time ago and it's water under the bridge as far as I'm concerned.
- Rhonda says her marriage to Mark is water over the dam and it's time to move on with her life.