I'm not sure whether I write it as if I'm reflecting back on what had happened or as if I'm presently going through the story?
You are procrastinating! :-)
I think you know that you can choose either of those. This is a creative assignment. So... that is why I think subconsciously you are procrastinating. That happens to me, too.
And it is okay if you have never written one of these. Just think of it as a speech! Before you begin, google this:
Gregor Samsa character analysis
That will help you to know what feelings and ideas you want to "speak" about.
Good luck!!