I have struggled with discipline a lot in the past. And even though I think I've solved that problem there are still moments where I feel nothing's working.
I begin a script on impulse most of time. It's like a combustion process, it just propels me strongly enough not to need extra discipline. So if I have inspiration, it's easy to ride on a scene. It lasts hours and I'll stay up all night writing it out. But If can't write a passage in one shot and I need to finish it the next day, then I lose my momentum and start to panic and eventually call it a day.
Unfinished business is a serious bummer and a killer. This is why I've trained myself to write on demand and I must say I'm happy I did.