How can one get rid of the habit of masturbation that has accompanied the questioner since childhood, despite his repeated attempts to quit it, and its impact on his religious and worldly life?

The mercy of Allah is vast, and what you feel as an inability to repent is a deception from Satan. Your abstaining from sin for long periods is proof of your ability to abandon it. You must supplicate frequently and sincerely turn to Allah to remove this affliction from you. Whenever you fall into sin, repent and do not tire of repenting, for Allah accepts your repentance no matter how often the sin is repeated. Do not surrender to Satan's temptation, for no matter how many times you repent, Allah erases the trace of the sin. Continue with good deeds and supplicate for ease in marriage, for "indeed good deeds do away with evil deeds."