Waste not fresh tears over old griefs

शोको नाशयते धैर्यम्
शोको नाशयते श्रुतम् ।
शोको नाशयते सर्वम्
नास्ति शोकसमो रिपुः ॥

- रामायणम्

Grief destroys one's courage. It takes away his knowledge. It destroys all good qualities of a person. Definitely, there is no enemy comparable to grief.

Let bygones be bygones.
What's done cannot be undone.
Waste not fresh tears over old griefs.