Your school subject is not college.
I've done several things in my life that I wish I hadn't done.
I feel guiltiest about pulling a chair out from under my best friend when I was 12 years old. The problem is that she was in a body cast from her hips to her shoulders. Fortunately, she wasn't injured, but I've spent over 60 years feeling guilty about that stupid and unthinking act.
The prompt is asking you to think of any decision that you later regretted, and wished you could have changed. Then propose what would have happened had you made the better choice.
To answer, look at your personal choices. You should try exaggerating a real experience. If you cannot find anything of relevance, just make something up. The analysis is more important than the event itself
I don't believe that you need to make something up. We all have done something we have regretted. However, because of our feelings of guilt, we often do not have the courage to examine our behavior. Even if we can examine the behavior ourselves, we may not want to share it with others, for fear of being rejected or judged.
You can consider a sin of commission (something you did) or a sin of omission (something you failed to do).
I hope you can muster the courage to face your actions (or lack of them). Examining your actions out in the open typically reduces their emotional impact.
I hope this helps a little more. Thanks for asking.