You just have to accept every project will have low points where you want to quit. This is generally at points of uncertainty, like you're forced to draw something you're uncomfortable with. You just have to draw through it, even if it means take a break for a while studying the subject you're uncomfortable with before returning to the project with refreshed vigor.
There's two things. One is that you have to know your limits. If you have never managed to make a comic longer than 3 pages long, or written more than a prologue, your 9-volume epic project is probably not going to become a reality. Aim for a complete minicomic or short story and work your way up.
The other thing is that you have to really actually want to finish the project. You might have a deadline and want to finish it and get it over with, or you might just want to hold it in your hands really badly. If you want to finish it you'll keep reminding yourself to work on it. If you don't care very much you'll likely dump it as soon as it becomes difficult.