@Nevetharine I don't think it really matters if you write for others or not. What I think matters is your intentionality. If you do something when you feel in the mood, or when motivation/inspiration strikes, your progress will be slow and erratic. You'll lose the flow and the interest. (I am using the pronoun 'You' in its universal, generic form, not specifically to identify with the individual we all know and love as Nevetharine!).
Occasionally I have come back to something I've written and wondered what on earth I had intended with that comment. There'd been a time gap and now I had forgotten. I don't like being in that situation so I make sure it no longer happens. Sure, there's a break between drafts, but the work is in itself complete albeit perhaps not tight and tidy.
Even when writing for myself (and there are some children's stories written from the pov of my chickens that will
never see the light of day though my little girls loved them!), if I think of it as a job and one that I actually enjoy, then I'm more likely to ensure that there is appropriate time and effort and consistency.
That's me, of course, and yeah, it seems to work pretty well. And when for whatever reason I can't write, I'm doing research or studying the craft. And that seems to be what you do too, by writing something every day. Just like going into the office (or wherever) every day. Some days you might slack off, but you rock up every day anyway.
And I don't think any boss would accept it if you phoned in and said, "sorry, I've got clerical assistant's/lawyer's/salesperson's block and can't come in today."!