I understand how you feel, but just start writing it, you will be fine. I have already written three books this year and I experienced the same feeling you had, but when I just started, the follow was easy.
I actually rounding up my third book now has I make this comment.
THREE books this year...?
Holy shit, mate! You’re on fire! 🔥
Fiction / non-fiction? What subjects?
Yah, hahaha!!! They are non-fiction books.
They are all relationship books about the relationship btw a man and a woman of different aspects.