Dallas Willard writes:
"It is not the growth of “will power” we are looking for in spiritual formation, but transformation of all dimensions of the self under the direction of God, through a will surrendered to Him and applied appropriately to bring about personal change." (Dallas Willard, Getting Love Right, Kindle Locations 222-226)
We are not to work harder or try harder to self-transform into Christlikeness. To think that trying harder will achieve, e.g., the kind of love Jesus had is to devalue Christ and diminish expectations. "Will power" (Richard Foster calls it "will worship") won't work.
But as we surrender to Him He transforms us. He produces spiritual "fruit" in us, which are qualities of His own being. As we abide in Christ He gives us precisely what we cannot achieve in our own strength; viz., the ongoing meta-morphing of our heart into a heart like His.
(Click on the link and get Willard's beautiful little book, on Kindle, for $1.)
(On the powerlessness of will power to conquer addictions see Addiction and Grace, by Gerald May.)