Many great suggestions here already. As a life-long creative/solo-preneur (musician, author), I can relate. Focus is the single biggest factor for me, too.

The one suggestion I would add, which is also the highest leverage thing you can do, is work to develop better cognitive and impulse control.

It's great to remove distractions and get your priorities straight, as others have suggested. But it's much better to make yourself distraction- and stress-resistant, which is what cognitive control training gives you.

As a bonus, it also solves the other inner game problem entrepreneurs tend to have, namely that their life looks better on the outside than it feels on the inside.

Best of luck! Let me know if I can help in any way.