What are the legitimate (Shar'i) reasons that help a Muslim to be patient with Allah's painful decrees, and how does one repent in detail from inner discontent (tasakhkhut qalbi)?

Patience does not negate sadness or the wish for an undesirable situation to cease. The requirement for patience is to restrain the tongue from complaint, the heart from resentment, and the limbs from actions indicative of distress. Whoever is distressed by an affliction must repent by ceasing the distress, regretting it, and resolving not to return to it.

Among the things that aid patience are: Contemplating the immense reward of the patient. Recalling the greatest calamity: the loss of the Prophet (peace be upon him). Studying the biographies of patient individuals, including prophets and righteous people. Knowing that sadness does not alter Allah's decree. Knowing that this world is not a place of permanent comfort. Knowing that Allah is wise in His decree, and that His wisdom holds secrets we may not comprehend.