My favorite saying is "I can be what I will to be"--Charles Haanel.
But, twin studies do seem to show a common gene among addicts. There's a predisposition.
So, I use my free will to manage my genetics (my inclination toward numbing out).
My daughter is predisposed to addiction. She is using her free will to never start drinking (and I really hope it stays that way).
Changing the environment, which is premediated through thought, is very important, no doubt about it.
I think free will is the most powerful thing. My daughter is vegan and surrounded by meat eaters. She couldn't change the environment but due to her love of animals, she uses free will to be vegan (going on 4 years now). This is a girl who loved her steak raw.