Did everyone miss the obvious? You talk about writing during your working day. You don't have to, every day. Write on your day/days off when you're not as tired. When I worked full time, I used to write maybe three times a week, for maybe two hours a time. And I've written over 20 books, ten of them published.
You want motivation? Just start. If you write rubbish, it doesn't matter. What is important is that you will have something to re-write, and that very act will set you off, even if the act or rewriting becomes tearing it up and starting afresh. I wonder if it's motivation you need - if you really have a desire to write, you will - or whether it's a dearth of ideas & there are ways round that problem.
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