How can one get rid of the feeling of constant fear of Allah and unworthiness of a decent life, after repenting from sins of adolescence?

No matter how great the sins, Allah forgives and erases them with sincere repentance. The one who repents becomes as if they had no sin, for the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin."

One should not despair of Allah's mercy, for Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'"

Therefore, one must turn to Allah, have good expectations of Him, frequently seek forgiveness, and fulfill the conditions of repentance: abandoning sins, regretting them, resolving not to return to them, and restoring rights to their owners if they pertain to others.

Similarly, it is recommended to increase acts of obedience and good deeds, "for indeed good deeds efface evil deeds," in order to begin a new life with a tranquil heart, confident in Allah's pardon and mercy.