How can a young person returning from the path of straying and falling into sins repent and return to prayer, and feel that their repentance is accepted despite the multitude of their sins?

Committing sins and persisting in them destroys the soul. You must awaken from your heedlessness, for there is no barrier between you and repentance except overcoming Satan and the evil-commanding self, and proceeding towards your Lord with steadfastness and sincere determination. If you are sincere in seeking repentance, Allah will grant you success, as He Almighty said: "And those who strive for Us – We will surely guide them to Our ways."

And to become upright: 1. Strive sincerely against your self by observing your obligatory duties and abandoning forbidden acts immediately. 2. Change bad company for the company of righteous people. 3. Adhere to remembrance (dhikr) and supplication (du'a), and ask Allah for help and steadfastness. 4. Reflect on the consequences and danger of sins, and that they invite Allah's wrath. 5. Reflect on death and the horrors that follow it, and that it is near and discriminates against no one. So beware lest it overtakes you while you are engaged in these forbidden acts, leading to an evil end for you.