This is a new favorite quote of mine: "Here's the paradox: you will not be able to truly write your book unless you have written your book. GET. IT. OUT. OF. YOUR. HEAD. Everything will probably change later- that's fine. At this stage, accept deep imperfection." -Jessie Burton
And another, because you know, motivation. "I think new writers are too worried that it has all been said before. Sure it has, but not by you" -Asha Dornfest. 
A tip: Write for you. When you write for yourself, that's when you truly start to believe.