Question: You say that only Hashem can give us ultimate victory over lust. Then, where does human willpower and bechira come into play?
Here are a few good resources that can help us understand this sticky topic:
1) This PDF booklet discusses the issue in depth.
2) A great article on the topic: When He removes His help, we fall.
3) Principle 17 - Don't Dwell on the Past
4) An amazing take on the story of Yosef and the wife of Potifar