This happened to me when I first started writing my book too. I am always in the constant state of lethargy and procrastinating is a daily occurence to me, even to this day. The thought of 'let's just do it tomorrow' or 'the ideas aren't coming up to mind' is the excuse I use to not finish the book.
But for me, what motivates me to keep going is the thought that if I don't finish this now, I could never finish my next book. Because now that I finally released my book, I realized that writing isn't that hard. Once you finish one, the thought of 'let's do it tomorrow' becomes 'let's write a little today' and the excuse of 'the ideas aren't coming up to mind' is just me overthinking that my ideas aren't interesting enough even though I have a lot of ideas to pour into words.
In my opinion, you just need to find the flow, create the scene of each chapter in your head before you start writing, and lower your expectation. Even if no one read your book, you shouldn't be disheartened, especially if it's your first book. Just write what you want without being burdened with what the reader would expect from you.
But yeah, that's just my way of writing as an amateur author, maybe other experienced author can help with more tips. Anyway, good luck with your book. Cheers!