Shaytaan is full of tricks, making us sin over and over again. A person after having sinned and sought repentance, commits again the same sin after a short while, purely by mistake; he then repents. A few days later he sins again and then repents, he continues doing this. Then shaytaan puts doubts into his head: ‘look forget all this, it’s not worth it, you will never stop this sin.’
No!! Carry on repenting for Allah is All-forgiving. Mujaahadah is like a boxing match, when you have done a sin you have fallen, but as soon as you have repented by istighfaar you are standing up again, before the count of ten, and you are back in the game.
If you have to repent a thousand times a day then do it, but don’t give up, why? Because through the barakah/blessings of tawbah Allah will help you give up your sins! It is our job to make a firm resolution and to do mujahada of our nafs until it becomes weak and our slave, we have to show Allah ta‘aalaa that we are making a sincere effort, then insha’aallah, He will grant us the tawfeeq (ability) to give up sins.