What should I do, as I swore to abandon a sin and fast for one day whenever I returned to it, then I returned to it and fasting became difficult for me, and I desire to repent?

If you swore not to commit a sin and to fast for a day if you did, and you committed the sin but did not fast, then an expiation for breaking an oath becomes incumbent upon you. If you vowed to fast for a day whenever you committed a sin, you have the choice between fulfilling the vow or offering an expiation for breaking an oath, which is feeding ten poor persons, clothing them, or freeing a slave. If you are unable to do so, then you must fast for three days. The aforementioned sin—if it is masturbation—is forbidden and invalidates the fast if committed during the daytime in Ramadan. What is incumbent upon you is sincere repentance from sins, striving against your own desires (jihad al-nafs), and seeking Allah's help.