Write your memory of that night. Describe the house where that event took place. Make a list of all you learned from what happened. Do a brain dump. Write 500 words.
Would it help to demystify the process, writer? Writing a book is essentially an act of completing one small task after another.
So when the whole of the work overwhelms you, break it down. Give yourself a series of mini assignments. And when you finish those, give yourself some more.