seal (one's) fate
To solidify or confirm that a particular, usually unpleasant, thing will happen. If you fail this exam, it will seal your fate and make your GPA low enough to qualify for expulsion.
seal one's fate
Decide what will become of one, as in The letter of rejection sealed his fate; he'd have to apply to other medical schools. This term employs seal in the sense of "permanently fix or fasten something," a usage dating from the mid-1600s.