How can one strengthen trust in Allah and supplicate with certainty of response, and does doubt regarding non-response or its delay contradict reliance on Allah and certainty of response?

A Muslim should understand the jurisprudence of Du'a (supplication) from two perspectives: The first relates to his heart, for he must be certain of the response, hopeful of Allah's generosity, and mindful of the vastness of His bounty, as the Prophet, peace and blessings be upon him, said: "(Supplicate to Allah while you are certain of the response)." The second perspective relates to the Du'a entering into Allah's decrees and wisdom. Allah has profound wisdom in His creation and command. The Du'a may not be a direct cause for warding off evils; rather, Allah may save it as a reward in the Hereafter or avert another harm with it, as the Prophet, peace and blessings be upon him, said: "No Muslim supplicates to Allah Almighty with a supplication in which there is no sin or severing of kinship ties, but Allah grants him one of three things: Either He hastens his supplication for him, or He saves it for him in the Hereafter, or He averts from him an evil equivalent to it."

Ibn al-Jawzi opines that the non-response to a Du'a might be a trial requiring patience, and it removes the whispers of the nafs (self). There might be wisdom or benefit in the delay of the response, or it might be due to a defect in the supplicant, or because the attainment of the requested thing would cause harm, or because the loss of what is lost becomes a reason to turn to Allah. The true believer's hope and expectation remain firm even if the response is delayed, for he knows that Allah knows best what is beneficial, and that what is intended from him is patience and supplication.