Personally I think it's saying that his teachers told him he could do anything if he applied himself to it (And all the teachers used to teach
You can do anything if you put your mind to it). He tried his hardest, but expected things to happen right away and quit (We put our minds to it all
But disappointment crashed the ball). He let it go and things started turning out kind of poorly for him. He ended up regretting having not waited (We could've done anything We just never quite knew it). So it's kind of a sad song about his regrets in life. Or at least the regrets of the person's point of view that Passenger was writing from.
Personally I think it's saying that his teachers told him he could do anything if he applied himself to it (And all the teachers used to teach You can do anything if you put your mind to it). He tried his hardest, but expected things to happen right away and quit (We put our minds to it all But disappointment crashed the ball). He let it go and things started turning out kind of poorly for him. He ended up regretting having not waited (We could've done anything We just never quite knew it). So it's kind of a sad song about his regrets in life. Or at least the regrets of the person's point of view that Passenger was writing from.