Will Allah forgive a person who associated partners with Him knowingly, then repented and desired to change his life, and how will he be forgiven, and how will he know that, and how can he strengthen his faith to do what is lawful and abandon what is unlawful, while having psychological problems that lead him astray?

Indeed, Allah forgives all sins for whoever repents to Him and returns, even polytheism (shirk). And whoever repents, Allah accepts his repentance. No matter how great the sin, Allah’s pardon and generosity are greater. The servant must turn to Allah, regret what he has done, and resolve not to return to it. For "Islam demolishes what came before it," and "the one who repents from sin is like one who has no sin." To strengthen faith: increase in the remembrance of Allah, the recitation of the Qur’an, and prayers upon the Prophet. Maintain the obligatory prayers and the supererogatory ones, accompany the righteous, read the biographies of the pious, and distance yourself from everything that reminds you of sin.