Hazrat Moulana Ashraf Ali Thanwi (rahmatullahi ‘alaih) once mentioned:

The jihaad of the nafs in certain respects is even more difficult than the jihaad against the enemies on the battlefield. The reason for this is that when one fights against the kuffaar, despite it being an exceptionally great sacrifice, however very quickly one is slain through the sword and earns the rank of martyrdom. On the contrary, the jihaad against the nafs is extremely difficult. Every moment one has to perpetually oppose the nafs in fulfilling the orders of Allah Ta‘ala and refraining from His displeasure. (Malfoozaat Hakeemul Ummat 5/206)

Source: Ihyaauddeen.co.za