Can you transition well?

Transitions.  I can't do them.

How the heck do successful writers who are parents manage to transition from working on their novel to being the person who has to pick up the kid at school on time?

Here's the scenario: I'm hard at work on a section that has been impossible, a scene in which this not-really-out gay kid turns away from the guy who ought to become his lover and instead does what he thinks society wants - he walks away. It's a scene that has been impossible to write, it's painful, it's obviously the wrong thing to do...but today, after gazillions of cups of coffee and utterly ignoring all the laundry in my home, I finally got it to work. Sort of. At least, it began to work.

and then the timer on the microwave beeped, reminding me that I had to pick up my daughter from Superhero Club in 20 minutes.

Is that fair?

I couldn't drop the scene. Heart pounding and knowing that she would level me with all the guilt tripping she learned in Kindergarten, I typed faster. I wanted the scene on paper (ha, paper! in memory, I guess) and so I zoomed through a skeletal mockup.

but here's the trade: now it's not pressing at me. It's gone. Out. It's no longer urgent. The next time I get to go back to it, I don't know if I'll be in the same place, creatively. It's a horrible loss--or perhaps--a potential loss that could be horrible.

So how, people of the blogosphere, do people deal with this? Ideas? Set the timer earlier? Later? Don't beat yourself up? How?