Does my insistence on supplicating to God for a matter in which I have not yet succeeded, after performing Istikhara and placing my trust in Him, count as an objection to God's divine decree and destiny, and what should I do?

Continuing to supplicate is not prohibited; rather, it is one of the strongest means of attaining what is desired and averting what is disliked. However, if you become certain of the impossibility of what you are supplicating for, then you must accept Allah's decree and cease the supplication, because supplicating for the impossible is considered an transgression, and Allah does not love transgressors.