I try to compartmentalize. Sometimes I fail.
Back when I first started writing, I thought I was busy, because I was studying for my Ph.D. Really, though, what I was doing was switching between two head spaces - the studying one, and the one associated with the trilogy I was writing.
Looks so simple now.
After I had my first baby I had to re-boot the system and figure out how to fit in writing again. I got the hang of it. I had my real life children-motherhood mode and found tiny places where I could have enough room to escape into a small bubble of my old head space. I gradually grew those bubbles bigger until I could alternate more.
The more I write, though, the more head spaces I need. My writing depends on being able to immerse - to put myself deep into a place that isn't at all like the way I would ordinarily think. Recently as I've been writing For Love, For Power, I've been reaping the rewards of putting myself so deeply in: small moments are happening in the text that jump out to me as "real." Not only as things that are likely to happen in my caste system, but ones that would happen all the time there, and whose significance isn't exaggerated, but which fit precisely into the whole context - like when a servant replies to a question with "I couldn't say, sir" and my noble boys glance at each other with a shared look that says, "yep, he's under oath."
The hardest part is that I have more than one story I need to write. Three, in fact. And the head space issue is precisely the same for each one. There isn't a story that I can write by just skimming along the surface. I have to give myself to it, or it doesn't happen. I try to build in time that lets me drag myself out of one and put myself into another, but it's still hard. Yesterday the novel was rolling, and I could hardly manage to get back out again into real life, much less give any thought to the other two stories I'm currently working on.
I don't think I'd change this. I don't want to start skating on top of a story. I'm just going to have to decide when to take a breather from one and engage the other, or the other. The deep head space is that place I always loved going when I read books as a child. That I can find it for my own stories as an adult is exciting and rewarding - worth the sacrifice. I just keep trudging along, figuring that somehow it will all get done, and that keeping the balance will get easier.
Good luck with all your writing today. I hope you find that deep head space that I love so much.