How can one advise a person who was religiously committed but is now struggling to stay away from sins and has abandoned some acts of worship, knowing that he is striving hard to resist his desires and feels remorse?

The advice for this brother is to make a sincere repentance, which includes abandoning the sin, resolving not to return to it, and feeling deep remorse for what he has committed. What helps him in repentance is contemplating the ugliness of the sin, imagining his standing before Allah, knowing that the Book leaves out neither a small thing nor a large one without recording it, reflecting on Allah's awareness of him, and that he should fast frequently, and adhere to supplication, remembrance (dhikr), and seeking refuge in Allah. He should also associate with righteous people and avoid the circumstances that lead him to sin. If he returns to sin, let him repent anew, and let this be his consistent practice. This will not harm him as long as he sins, then seeks forgiveness and repents.