When talking about focus, there tends to be a spectrum between scattered and obsessed. When you are scattered, you have a bazillion stories that you are working on simultaneously in your head. When you are obsessed, you have one story. One idea. For months. Maybe years.
Obviously this doesn't cover all cases, but in general it is a good system of classification.
So the trick is to break out of what you are and do something new to freshen things up. So if you are scattered, pick one idea, nail it down, and make a goal to see it finished. If you are obsessed, put down what you're working on, and try something new.
Writing Exercise: Write down ten ideas. If you are closer to the scattered, write down ideas you've had. If you are closer to obsessed, write down new ideas. If you are closer to scattered, pick one of those ideas, and make a goal to finish it. If you are obsessed, try writing one of the new ideas.
In the end, once you've mastered your brain and all, you'll probably want a type of mindset called relaxed focus, or "scattersessed" in which you have one main idea and a bunch of others floating around in the semidistance. Not close enough to interfere, but close enough to get the attention that they deserve.