Nafs

Some of the Salaf (rahimahumullah) said:

I battled with my nafs for twenty years before it became upright. If you strive for long enough, and repent each time you err, then your nafs will become upright. But, this is something that doesn't happen over days or months; it requires long years.

[Hilayat al-Awliya’, Abu Nu’aym (rahimahullah), 7/5]

Note: this does not mean you do not strive because it takes so long, it means you strive and seek aid from Allah and repent every time you err.