I've wrestled with this question from time to time and, to the extent I've found a solution, the key is duration:
In the short term, choices are mutable (but determined by character), and character is fixed.
In the long term, character is mutable, and determined by some inner nature plus the integral of experience, influence, and choice.
There's still a recursive element here, because the investments you make in yourself are strongly influenced by a prior version of that self. (In a limited sense, this resembles coherent extrapolated volition.) Still, I've found it more helpful to think about short-term vs. long-term processes - in the sense of stock vs. flow, intercept vs. slope, or short-term vs. long term elasticity - than about recursion. Adding time conveys an intuitive understanding not only of what you can and can't change, but what you should and shouldn't.
To phrase it a simpler way: willpower is a muscle, as are most other traits. Most people can be strong, but not all at once, and when you see someone lifting a heavy weight it means they've been training for a long time.