NFL coaches say that the first points in a game are always the hardest to get. The same is true of writing a book. The first few pages can establish momentum, but getting those onto paper can be excruciating. As another writer said: It's not like putting a hammer to a nail.
November is National Novel Writing Month (NaNoWriMo). Designed for would-be writers having trouble overcoming inertia, the project emphasizes quantity over quality: The idea isn't to write a masterpiece, just to write a novel in a month. Presumably if you've done it once, you can do it again; the experience of finishing a crude novel will make the goal of a decent novel seem attainable.
The average screenplay will consist of about 60 scenes. So if you write one scene every week, you can finish a movie every year. You don't need to write them in order; skip around, and fill in the gaps as you go. The point is to break up a colossal undertaking into manageable pieces; and any method will do, whether it takes a month or a year. You may not end up with a masterpiece; but you may acquire the confidence to bring that masterpiece within reach.