If you don't believe in what you do, you will give up at some point. If you have no reason to believe it's possible to achieve your dreams or goals, all the effort you are putting into it will be wasted.
You mind’s unconscious beliefs play a significant role in the amount of effort you put into your life's work. If you don't see a successful outcome, you won't push yourself further to get there. Your progress depends on your decision to try knowing that you will overcome your failures and rise above them.