If you don't have the balls to deliver something, don't make promises.
Over-promising your performance to an employer will only get you fired, even if you managed to trick the company into hiring you. Over-promising your commitment to a cause to gain people's favor will only lead you to looking like a fool in the end. Over-promising your love for a person to gain their affection will only lead to bad karma if you're successful in your pursuit just for the person to find out you've cheated them their feelings.
Don't even try to give the excuse that you don't know yourself well enough. Grow up! s*** will always be s*** no matter what angle you look at it.
You'll always be a loser for lying to everyone. Don't lie or over-promise and under-deliver!