I hear this from many of my students while they are in the middle of writing their first ebook. It is perfectly normal to feel this way, especially while you are in the middle of writing. At some point you get muddled down with the process of writing and you feel that what you are writing is not as important as you originally thought it would be. Fear sets in, and you feel like what you are doing is just a big waste of time and energy.
When you feel this way it may be a good time to step away from your writing for a day or two. Do some other things and do not even think about your ebook. Then you can pick up again with a fresh perspective and point of view. Know that what you write is valuable and will be beneficial for the many people who read it. Keep the end goal in mind and press onward with your ebook writing.