The only real failure in life is not to be true to the best one knows.

There is a difference between falling short of a result and acting below what you understood at the time. The first happens constantly and is mostly outside your control. The second leaves a residue, because you were present for it and you knew. Judged this way, a defeat where you did what you believed was right is not a failure at all, while a comfortable win against your own judgement is.

At the end of a hard day, ask whether you acted according to what you actually knew, rather than whether it worked. That is a question you can answer honestly, and the only one you have full information about.