How do you know the result of Salat al-Istikhara, and does a person have a role in their destiny, especially concerning marriage? And is the failure to complete a marriage after it has begun a trial (from God) or a result of human actions?

The outcome of Istikhara (guidance-seeking prayer) manifests in what Allah decrees for the supplicant, either by proceeding with the matter or turning away from it. Thus, what happens after Istikhara is Allah Almighty’s choice, whether the desired thing occurs or is averted. Everything that transpires in the universe is by Allah’s decree. The servant has a will and an intention with which they choose their actions, but these are subject to Allah's will. The servant is commanded to perform Istikhara and to take the necessary means, and they must be content with what Allah has decreed, knowing that Allah possesses profound wisdom in all His judgments. What a person dislikes might be a test, or due to sins, or for a benefit unknown to the servant, and they must submit to Allah's divine decree and destiny.