What is needed is the determination to follow the chosen course. Some believe that our willpower is very much like a muscle.
فَقَدْ عَرَفَ مَا أَرَادَ ٱلْقِيَامَ بِهِ أَوْ مَا يَجِبُ فِعْلُهُ ،‏ إِلَّا أَنَّهُ لَمْ يَتَمَكَّنْ مِنْ ذٰلِكَ فِي بَعْضِ ٱلْأَوْقَاتِ .‏