@nassos Nine and sixty ways...
There's a Puritan ethos that says 'writing should be hard work' and also 'stick one project through, no matter how difficult', both of which led to my having a ten year writing hiatus on a novel I wasn't ready for, which is also the only one I ever outlined.
Now I'm revising some, admittedly, terrible, terrible novels, but they're at least in second pass. I've learned to fix my own bad habits, and the new ones are noticeably better. And I'm enjoying all of it.

@nassos I can only write/rewrite one fiction thing at a time.
I can write/write one fiction thing while also dealing with editor or beta reader notes on another fiction thing.
I can also write/rewrite one fiction thing while also working on writing/rewriting a non-fiction thing. Like morning do fiction and afternoon non-fiction.
I tend to work on one thing at a time but I often have projects in various stages of production.
Right now:
An essay series on #SelfPublishing and marketing
3 novels in 2nd draft
1 novel in planning/1st draft
My most active project is building a robust story bible for the 20 books I've published.