Make the best of a bad bargain.

When an agreement turns out poorly, the practical response is to recover whatever value remains rather than dwell on the loss.

The proverb also applies to an unwelcome situation that cannot easily be changed. It encourages a person to work toward the best outcome still possible.

Use it to encourage someone disappointed by a deal or other unfavorable outcome to consider their next steps. It fits when the immediate problem cannot readily be undone; it does not imply that the original terms were fair.