Is everything that occurs in the universe, good or evil, willed by Allah for a divine wisdom? Does Allah prevent a servant from falling into error, or does He leave him to choose? And can what a person believes to be evil for him actually be a good willed by Allah?

The mental fatigue you feel as a result of contemplating divine decree (qadar) is natural, because qadar is God's secret among His creation, and delving deeply into it is a path to disappointment. Allah Almighty has not revealed His decree in detail to anyone, and whoever asks "Why did He do it?" has rejected the ruling of the Book. The believer's duty is to believe that Allah Almighty is wise in His actions, all-knowing, and that He possesses the conclusive argument. If you believe this without delving deeply, your heart will find rest and your faith will become stable. So, refrain from delving into matters of divine decree, and believe in what the Qur'an and Sunnah indicate, and you will find comfort, God willing.