How can one get rid of the contradiction between outward observance and falling into sins, especially with leading people in prayer, and addiction to masturbation and pornography, along with weakness of will and fear of a bad end?

Treating a decline in one's spiritual state requires sincere will and determination, and it begins with repentance to Allah Almighty, through remorse, seeking forgiveness, abandoning sins, and resolving not to return to them. One must have a good opinion of Allah, humble themselves before Him, and avoid despairing of His mercy. It is recommended to seek Allah's help, increase acts of obedience that expiate sins, observe the five daily prayers with humility, take righteous companions, and turn to the Book of Allah through recitation and contemplation. One can refer to books on spiritual edification for further benefit.