How can one find contentment and forgetfulness after becoming attached to the hope of a specific supplication being answered, only for the situation to suddenly change? And what is the wisdom behind that?

Supplication (Du'a) is a great act of worship that has three forms of response: either the fulfillment of what was requested, or its storage for the hereafter, or the warding off of an equivalent evil. There are no specific legal signs for the acceptance of supplication, but some scholars have mentioned empirical signs such as humility, weeping, and tranquility of the heart. Visions are not definitive proof of the acceptance of supplication, as they may be from Allah, or from Satan, or mere self-talk. A Muslim should submit to Allah's wisdom in managing affairs, and not delve into the secret of destiny, and entrust matters to Him, and repent, and remain steadfast in obedience, and frequently engage in remembrance (Dhikr) and supplication.