Earlier today in a comment I tried to explain to a community member that they should put whatever happened in the past behind them. I used "water behind the bridge" and "let bygones be bygones". For context, the two English idioms are:
—used to say that something happened in the past and is no longer important or worth arguing about
We had our differences in the past, but that's all water under the bridge now.
to forgive someone for something done or for a disagreement and to forget about it
I know we've had our fights over the years, but I think it's time we let bygones be bygones.
I've found some explanations of the two phrases in Japanese but have yet to find a corresponding ことわざ.