So this comes up fairly often in language learning forums. Language acquisition is an especially good example since it can take quite a long time to get to a decent point of proficiency, and often times external rewards are minimal. So finding ways to carry on through the long, rough periods is essential.

Motivation is essential. Why else would anyone bother, except for either a driving need or a deep and abiding interest? It's what keep you going when you start to question if you really want to keep at it. But motivation on a daily basis comes and goes. Everyone who goes to the gym or runs know that.

Discipline is about pushing through when motivation flags. I disagree that discipline is only useful for "closed mode". Discipline can also help you with open mode by making you daydream and play with ideas rather than packing it in for the day. Many extremely creative people have a very set routine. A writer might have fixed hours for when they write, every single day, for instance.

In the end it's about using both motivation and discipline together.