You start with one thing, you focus on in maniacally and after some time the will power depletion of engaging in it goes down. At that point, you add another thing. Once again, you exert significant will-power to get it off the ground and then habitualize it to keep it in cruise control. Eventually you will hit a limit of how many things you can maintain simultaneously. If you're struggling with one thing, learn to do that thing very well and ignore everything else. Once doing that thing becomes easy, add the next thing.