When you are trying to achieve something hard, do you need any more challenges to make it harder? Do you need to fight off doubt to achieve your goal if you didn’t have to? Would everything not be easier to achieve if you only fully believed you could do it, whether from the start or from somewhere along the way? Those are the reasons why you should believe, before you achieve.
The self-belief encouraged by this quote doesn’t have to be rational when applied. In fact, the more challenging an achievement might be for you, the harder it will be to rationalize to yourself you can succeed. However, beliefs aren’t supposed to be rational. You might know an adage in English touting That’s why they call it faith. That’s what the self-belief being encouraged is an example of, faith. Faith in yourself. Blind faith in yourself, if need be.
Now, I’m not encouraging reckless and completely blind faith here. I’m a big planner type. Do what you can to prepare, then leave the rest to faith. That’s when you should believe, before you achieve. Convince yourself, but don’t overthink it to rationalize it. Things will then be easier for you, whether you ultimately achieve your goals or not. You will, at least, have been able to give it your best shot for what you could control in yourself, without doubt getting in the way, and that’s a comforting feeling to have, both, before and after the fact.