Staying on a sinking ship is irrational behavior and will cause you to lose out in the long run. Cut your losses and admit defeat when necessary.
Don’t be afraid to admit you made a mistake.
This applies to all areas of life—don’t stay in an abusive relationship or continue on in a job that makes you unhappy. Don’t worry about being perceived as a loser who made terribly stupid investments—it’s more important that you achieve success in the long run.