How can one annul a vow to fast three days every time a person returns to masturbation, given that the days have accumulated to approximately 300 days, and it is difficult to fulfill the vow?

A vow that inherently implies repetition cannot be revoked unless the one who made the vow is completely unable to fulfill it; in such a case, he must offer the expiation for breaking an oath (kaffarat yamin). Striving against one's self to abandon sin combines the virtue of obedience, the struggle, and the fulfillment of the vow. Therefore, you must supplicate, truly rely on Allah, and seek His help.