If I am working for someone else, and I put in a lot of effort, then I feel completely drained after N amount of days (or after project completion) – which requires me to mentally "unlink" my brain from anything related to that project.
If I am working for myself, whether that be on my company, or for personal growth, then I can hit a wall day after day and still find motivation to continue forward, projecting even one little victory as something that can give me energy to keep going.
The key for me is motivation. If I feel invested (and rewarded) in my own growth, then I am much more productive than just adding more to the bottom line of some faceless organisation.
Good luck to you! I hope you find something worth while that both gives you energy and makes you feel like you're growing.