I'm right in the middle of a massive project of my own - I estimate that my current webcomic is going to take a total of 5-6 years to complete, and I'm only 2 years into it. Comics take time. It's the nature of the medium. You can find ways to speed up your work-process, but it's still going to take a lot of time.
Get used to the idea. It probably will take longer than you predicted, because you can't know if something else will get in the way, if your life situation will change, if you get sick, etc., etc.
I handle that strange bundle of restlessness, stress and boredom (working on any big project you will at some point get bored; it's just what happens) by doing other smaller projects on the side - oneshot comics, illustration projects, etc. Things that don't have the same hard deadlines and regular update schedule as my main project, but are still enough to let my brain play with the other ideas from time to time.
If you figure out how to stop your brain from doing that, please let me know! XD But jokes aside - yes, it is sometimes a good idea to try to stop your brain from going "what if I did x?" and just focus on the stuff you have. Instead of endlessly inventing new characters and worlds, try to pour that cretivity into the projects and ideas you already have!
When I'm in the middle of something, and a new idea pops up and demands attention, I write down some brief notes about it, maybe doodle one or two character designs, and then I stop. Just a short description and a few frills, and nothing more.
It's like putting a little mental sticky-note on that idea with the words DO THIS LATER on it. It means that a.) I satisfy the oooh-shiny-new-idea!-urge without actually launching a whole new project, and b.) it lets me actually have a useful record of my ideas, for when I actually have the time to do something with them.
It's easier to get those noisy ideas to shut up if you've actually written them down. Even just a basic list will do.