This is the 102nd most frequent Korean word.
자신의
"One's own" or "self's."
In this sentence, '자신의' is used to refer to something belonging to the subject 'he' — his own future.
그는 자신의 미래를 걱정했다.
He worried about his own future.
Here, '자신의' emphasizes that the homework belongs specifically to the student in question.
학생은 자신의 숙제를 제출해야 한다.
The student must submit their own homework.
In this sentence, '자신의' is indicating possession of experiences that are personal to the writer.