Shaykh Ahmed Zarruq (Allah have mercy on him) said,

‘Although the traveler (salik) must be more cautious after returning to sin he must not give up and despair. After every return, even a thousand time, he must renew his intention to repent.

If the Devil argues: “What is use of a repentance following a repetition of the sin?”, he must be answered, “Just as we made the sin’s repetition a habit, we make the repetition of repentance a habit too. ‘

Ahmad Zarruq, p282