the past is the past; let bygones be bygones
A set phrase used to accept that past events cannot be changed and should be left behind. Often used when forgiving, moving on, or refusing to dwell on old matters.
The past is the past. Let's move forward now.
That mistake is water under the bridge now, so don't worry about it.
A straightforward phrase composed of 過去 (past) repeated with the topic particle は, literally 'the past is the past'. No complex etymology.