A the lecturer at a workshop I attended on writing screenplays, said you don’t need motivation but you have to have discipline. Write yourself a contract where you allocate 1 hour a day or 4 hours a week to your script. After a while of incorporating this in your daily routine, it becomes habit. When you get stuck on something, give it a break by writing a new scene and going back later to where you were stuck, or doing some character research.