Audrey, that is really a fantastically great question. Not only is it a great question, it is a question that I deal with to this day, and almost every day. It is such a good question that I am going to give two answers, and an explanation.
Answer #1: There is no stressful part of writing a book.
Answer #2: There are many stressful parts of writing a book.
Explanation #1: We enjoy doing things we do well. We particularly enjoy doing difficult things when we do them well. Everybody agrees that writing a book is hard. Many people start out to write one, and just aren't able to get all the way there--and it doesn't get finished. They think, "someday I'm going to finish that book," but they never do. If you discover that you have the ability to put a whole book together, you have the right to feel good about it, even if it isn't a great book, even if no one is ever going to read it...still, you did it, and you sort of knew you could do it, and having done it, you know you can do it again, and really, it's a sort of beautiful feeling. Writing, once you get to doing it is a completely enjoyable experience. I have traveled all over the world, and had all kinds of adventures, met unusual people, seen wonderful things--and writing is more fun than any of that.
Explanation #2: All the stressful parts of writing a book have to do with thinking about other people, people who are not you, and not writing the book. Will anyone read my book? Will anyone like my book? Will some publisher publish my book? Is this going to be, (in the opinion of others), a good book? By that I mean wondering if your book will conform to the generally accepted, (that is, other people's), standard of what a book is supposed to be? Will my family and friends accept the idea that it is a good thing for me to spend all this time learning how, practicing, and trying to write instead of doing more usual and useful things? Stuff like that is stressful, and you have to guard against it, even when you are 75 years old.