"As many times as he has left, I have always taken him back."
This is why. We teach people how to treat us, through our actions. Breaking a pattern is difficult, but understand he has no reason, right now, to believe that this time will be different.
The good news is, he doesn't have to believe it for it to be true. You are the only one who needs to believe it.
What other support besides willpower do you have to help you maintain No Contact? Certainly you can come here and post when you are feeling less than strong, but do you have real life support of family and friends to help you through this? A counselor? Al-anon?