I got much better at this when my kids were born, because it was the only way I could get work done on some of my (computing) side projects. I went from having hours of uninterrupted "in the zone" time during evenings and weekends to having much less time overall, and what time I did have was broken into smaller chunks.

I got much more thoughtful about how I used my time and also got better at pre-planning what I had to do so as to make the best use of it. Mostly the key was to just try to tackle smaller tasks and accept that progress would be slow.

That's been exactly my experience as well. Sometimes doing a little research on a lunch break gives enough direction on how to spend available time later on my project.

Accepting that progress will be slow has been the most difficult adjustment, and applies to more than just side-projects. Choosing books or games also becomes a more strategic decision when what used to be a weekend sprint, turns into a several week marathon.