Robert Pippin (in "How to Overcome Oneself: Nietzsche on Freedom"):
But nothing so far cited has explicitly referred to self-overcoming or -mastery; the resistance in the quotation is not obviously inner or coming from one's self. Later:
…Nietzsche adds something that is easy to overlook.
How is freedom measured, individuals as in nations? By the resistance which has to be overcome, by the effort it costs to stay aloft. One would have to seek the highest type of free man where the greatest resistance is constantly being overcome.
Nietzsche here is most interested in a sort of psychological self-relation as constitutive of freedom … But what is this sort of self-relation? What counts as self-mastery in this sense? (pp 76f)
But the agency issues seem to me clearly present. This is so even though neither Nietzsche nor Zarathustra ever simply encourages us to "overcome yourselves." (The issue seems to be the proper acknowledgement and endurance of the self-overcoming character of life, an orientation that itself, as we shall see, has several social and historical conditions for its possibility.) (p 80).
Now Nietzsche, Gay Science 304 and 305:
304. By doing we forgo.—Basically I abhor every morality that says: "Do not do this! Renounce! Overcome yourself [überwinde dich]!" But I am well idsposed toward those moralities that impel me to do something again and again from morning till evening, and to dream of it at night, and to think of nothing else than to do this well, as well as I alone can! …
305. Self-control [Selbstbeherrschung]—Those moralists who command man first and above all to gain control of himself thereby afflict him with a peculiar disease, namely, a constant irritability at all natural stirrings and inclinations and as it were a kind of itch. Whatever may henceforth push, pull, beckon, impel him from within or without will always strike this irritable one as endangering his self-control: no longer may he entrust himself to any instinct or free wing-beat; instead he stands there rigidly with a defensive posture, armed against himself …
Let it be hastily said that Pippin can and does, of course, adduce plenty of Nietzschean texts in favor of his position and I am pretty sure it can be rendered workable in the light of GS 304 and 305: but it's rather remarkable that they are not mentioned at all in the paper.