Why must a person strive and exert effort in life, given that everything, good and bad, is already written and predestined for him? And is the feeling of being lost and not understanding religious matters, coupled with a sense of failure in various aspects of life, a symptom of jinn possession?

It cannot be definitively stated that this is a case of demonic possession (mass). A Muslim should strive for what benefits him, seek Allah’s help in his affairs, and exert all legitimate means, in accordance with the Prophet’s (peace be upon him) saying: “Be keen on what benefits you, seek help from Allah, and do not lose heart.” Supplication benefits against what has descended and what has not descended. Thus, a believer combines striving for what benefits him and seeking Allah's help, with the knowledge that whatever will happen is by Allah's decree. He must be diligent, strive, and take all necessary measures. The remedy for failure is to review one's standing with Allah through repentance and seeking forgiveness, having good expectations of Him and truly relying on Him, adhering to His piety, and being steadfast on His law. Everything that occurs in the universe is by Allah's decree and destiny: "Indeed, everything We created is by measure." Reliance on Allah is required, which means taking direct action while realizing that these actions only have an effect with Allah's permission. Therefore, exerting effort does not contradict reliance on Allah.