The question is, effectively, why pray for something that should be in our hands, if it's free will?
The obverse of this question is: when we pray that there be fewer sinners in the world, are we praying that G-d tinker with their free will? The Chazon Ish answered yes, we are. Rabbi Moshe Feinstein wrote otherwise; people have free will, but there are all sorts of exterior factors that will make it more likely for people to choose to do good (or bad). We pray that all those factors improve.
So I'd assume the same applies here. Not "dear G-d, please zap me so suddenly I'm unable to sin", but "dear G-d, please grant me presence of mind, good peers, healthy relationships, and the like, so I'm less likely to sin."
The Talmud observes that many people do what they want and go against the Divine will; but some people do things that are against both the Divine will and their own will! This can be caused by three things:
- Peer pressure
- Mental illness
- Severe poverty
Thus, observes the Sforno, Jacob prayed (when leaving home) "that G-d protect me on my path [from peers] ... give me food to eat and clothing to wear [not poverty] ... and I return in peace to my father's house [mental wellness]"; if all that happens, "then Hashem shall be my Lord."