Eggs cannot be unscrambled

नष्टं मृतमतिक्रान्तम्
नानुशोचन्ति पण्डिताः ।
पण्डितानाञ्च मूर्खाणाम्
विशेषोऽयं यतः स्मृतः ॥

- पञ्चतन्त्रम्

The wise never grieve over what is lost, dead or has passed. This is deemed an aspect that distinguishes wise from the unwise.

Let bygones be bygones.
Waste not fresh tears over old griefs.
What's done cannot be undone.
It's no use crying over spilt milk.
Eggs cannot be unscrambled.