When do you stop ?

Is there any way to know that you have tried enough? Is there a method to measure the level of how much you can try to achieve what you want / need ? If the end result of trying as hard as you can is always success, does it mean the failure did not try enough ?

Only when I start to give up, do I think to myself that maybe I didn't try hard enough. And the harder you try, and still fail, the more you feel horrible. The theory about trying as hard as you can and not worrying about victory or defeat is plain rubbish.

According to a friend, the only reason you lose is because someone else is trying harder than you are. But then when you try harder and harder, possibilities are that you might loose sense of reality.

I want to be determined, I want to be positive.... But I do not want to lose my sense of reality.