Is the fear of not succeeding in an exam due to sins or lack of effort considered a bad assumption about Allah? And can a person succeed after repenting from sins, or is a worldly punishment inevitable?

Fear of not succeeding due to sins and shortcomings in taking the necessary means is not a bad assumption about Allah; rather, it is a praiseworthy fear if it leads to repentance and making up for what has passed. If a servant repents with sincere repentance, then repentance erases what came before it, and the one who repents from sin is like one who has no sin. Allah does not punish him for the sins he repented from, as Ibn Taymiyyah said: "The one who repents is not punished, neither in this world nor in the Hereafter, neither by divine law nor by divine decree."