Achievements are the fruit of hard work, that's a truth that cannot be denied. Yet, can we say that it's the only factor behind it? Could we tell that with purely working hard we could achieve what we want?
Generally, the response is a hesitant yes, because depending on the circumstances we can't know if it will work out, since there are a lot of other factors that could influence our ability to achieve our goal, regardless of how hard you work. But how about knowing THAT, knowing (reasonably) that things can be beyond our control? What's its effect on us? On our work and motivation?
Well, the sad thing is that it's the first negative effect it could have on us, why? Because we listen to what other people say about it, how hard it can be, how likely it would end up in vain, and we decide to give up altogether or just working without the same conviction and motivation you started with, because now you know that (reasonably) you most likely won't get there since rarely anybody does.
Mostly our failures are related to the fact that we gave up too soon, and mainly we do that because we've lost confidence in our chance to get what we wanted. Knowing your chances and what you're bargaining for from the start is always a good bet, but you start letting it slow you down, then it just became burden weighing you down along the way and eating away at the energy you could be using to push forward.