Sakura is living* proof that no matter how hard you work..
Some people are born to succeed and others aren't, no matter how hard you try, if you're not meant for greatness you won't be great.
Naruto was meant to succeed, he was born from a great, well known clan, and a great, well known man, did horrible as a child, but due to his lineage and how he was meant to succeed, he grew into a great person
Sasuke was meant to succeed as well, born from a great well known clan, excelled without obvious effort as a child, grew into a great (power wise) person
Sakura, not meant to succeed, born from a not well known clan, she tried her damnest and was a star pupil as a child, grew into a worthless person.
*Not really living, shes just graphite**
**Yes I know they don't draw manga anymore, shes just ink