How to describe "choose to do something by one's own willing"
You can use the expression "You can do it of your own free will" or "You can do it of your own volition", or simply "It is voluntary", in other words, "It is not compulsory", to describe things that are volitional.
"By... own willing" does not even exist as a way of saying this. It is not a usable phrase, and you should eliminate it as something you would ever say or write.
To come close to the concept you want, you might want to use "of one's own accord," or "on one's own initiative."