What should one do who has been accustomed to masturbation and watching sexually explicit videos since childhood, and cannot stop them, and is unable to marry or fast to remedy that?

You must offer sincere repentance to Allah, for the one who repents from a sin is like one who has no sin. Repeat your repentance whenever you sin, and do not despair. Increase your voluntary fasting as a means of drawing closer to Allah. Associate with the righteous, adhere to remembrance (dhikr) and supplication (du'a), strive against your lower self (nafs), and avoid causes of arousal and sitting alone.