You're used to doing a lot of academic writing, which can be forced and dry. Loosen up! Begin by reading and understanding the prompt. Break it down by highlighting the key words and phrases:
Reflect on a time when you challenged a belief or idea. What prompted you to act? Would you make the same decision again?
This prompt has four elements. Next, simply brainstorm about each element. With this particular prompt, it would be a good idea to identify the belief or idea first, but don't worry about finding structure or making sense at this point (you'll get to that later!) Just write. You are on an exposition expedition! Don't judge, don't hold back, just keep your pen or fingers moving...for at least ten minutes on each element of the prompt.
Once you have exhausted your thoughts, print out your notes. That's right, print them out. Get on paper so you can get physical with what you've written. Now wade into your rivers of words and pan for the gold. Highlight what you like and what seems interesting to you. These gems will become the "bones" of your essay.