Is the continuous practice of masturbation, and at times without the ability to resist, due to magic, jinn, or an evil deed, or is it fate and divine decree?

There is nothing definitive regarding the cause you mentioned. However, it is advised to avoid imagining magic or possession, to fortify oneself with the prescribed remembrances (adhkar), and to resolve to abandon the prohibited bad habit. One must be diligent in getting rid of it by recalling God's omnipresence, occupying time with beneficial things, seeking help from righteous companions, and staying away from prohibited sights and idleness. Emphasis is placed on the importance of marriage and seeking chastity, as God has promised wealth to those who marry with the intention of chastity and obedience to God, in fulfillment of His saying: "If they should be poor, Allah will enrich them from His bounty, and Allah is all-Encompassing and Knowing." (Quran 24:32). Narrations and traditions from the Salaf (pious predecessors) have also confirmed that marriage is a cause for wealth and chastity for those who seek God's pleasure thereby.