"The person who fails the most wins."- Seth Godin
In the world of success and achievement, failure is more a matter of when than if. This means you always have to be prepared to fail.
The real question, then, is not whether you'll fail at an endeavor, but how big a failure it can be. How much is it going to cost you? How much are you putting in to (probably) fail?
In general, you want to invest very small, take as many small failures as you can, and learn from them.
For example, one of the things I do every day is play chess with a computer online. Sometimes I win, most of the time it ends in draws, but a lot of the time I get my ass kicked.
I've played probably a thousand matches so far, but because I keep playing and keep learning, I am able to eventually work my way up to higher levels of difficulty in the game.
This is not because I'm so good at winning, but because I've lost so many times. However, those losses don't cost me any money, any real emotional investment (pride, embarassment), and because I can learn quickly from the losses (how to escape certain situations, how to trap the opponent), I'm able to get to higher levels.
There's an amendment, then, we can put on the Godin quote: the person who fails the most, but is still in the game, wins.